What Is Klinefelter Syndrome?
Klinefelter syndrome is a common genetic chromosomal condition that occurs when a boy is born with an extra copy of the X chromosome.
Klinefelter syndrome can affect both physical and cognitive development.
According to the National Human Genome Research Institute, Klinefelter syndrome is found in about 1 out of every 500 to 1,000 newborn males, making it one of the most common chromosomal conditions. The Association for X and Y Chromosome Variations (AXYS), a group that focuses on Klinefelter syndrome and other similar conditions, estimates the Klinefelter occurrence at 1 in 600 male births.
Klinefelter Syndrome Causes
Klinefelter syndrome occurs as the result of a random genetic error during the formation of the egg or sperm at or soon after conception, according to MedlinePlus.
The syndrome is not inherited through the parents’ genes or the result of something the parents did or did not do.
Women who become pregnant after 35 years of age have a slightly increased risk of having a boy with Klinefelter syndrome, per the National Human Genome Research Institute.
People normally have 46 chromosomes in each cell. Two of those 46 chromosomes, known as X and Y, are called sex chromosomes because they help determine whether a person will develop male or female sex characteristics.
Females usually have two X chromosomes, while males have one X chromosome and one Y chromosome.
However, Klinefelter syndrome occurs because of one extra copy of the X chromosome in each cell (XXY), notes the Mayo Clinic. Because Klinefelter syndrome involves an extra X chromosome, it is also sometimes referred to as 47,XXY syndrome, notes AXYS.
Some males with Klinefelter syndrome have the extra X chromosome only in some of their cells. When this occurs it is called mosaic Klinefelter syndrome, per the Mayo Clinic.
While rare, a similar but more severe condition known as 48,XXXY can occur when a male has two extra copies of the X chromosome, according to AXYS.
Klinefelter Syndrome Symptoms
The most common symptom of Klinefelter syndrome is infertility, which often isn’t noticed until adulthood, notes the Mayo Clinic.
Klinefelter syndrome adversely affects testicular growth, resulting in smaller than normal testicles, which often leads to low production of testosterone.
Although most men with Klinefelter syndrome produce little or no sperm, reproductive procedures may make it possible for some men with the condition to father children.
The effects of Klinefelter syndrome vary from person to person, and signs and symptoms of the syndrome can appear differently at different ages, according to the Mayo Clinic.
Symptoms in babies:
- Weak muscles
- Slow motor development of things like sitting up, crawling, and walking
- Speech delay
- Docile personality
- Testicles that haven’t descended into the scrotum
Symptoms in boys and teenagers:
- Taller than average stature
- Longer legs, shorter torso, and broader hips
- Absent, delayed, or incomplete puberty
- Less-muscular bodies and less facial and body hair after puberty
- Small, firm testicles
- Small penis
- Enlarged breast tissue
- Weak bones
- Low energy levels
- Shyness
- Difficulty expressing feelings or socializing
- Problems with reading, writing, spelling, or math
- Difficulty being attentive
Symptoms in men:
- Infertility
- Small testicles and penis
- Taller than average stature
- Weak bones (osteoporosis)
- Decreased facial and body hair
- Enlarged breast tissue
- Decreased sex drive
Complications
Men with Klinefelter syndrome may be at increased risk for the following:
- Varicose veins and other problems with blood vessels
- Breast cancer and cancers of the blood, bone marrow, or lymph nodes
- Lung disease
- Autoimmune disorders, such as type 1 diabetes and lupus
- Belly fat, which can lead to other health problems
- Testosterone replacement therapy can reduce the risk of the health problems listed above, especially when therapy is started at the onset of puberty.
Klinefelter Syndrome Diagnosis
If you suspect that you or your son may have Klinefelter syndrome, your doctor will examine your (or your son’s) genital area and chest, as well as conduct tests to check reflexes and mental functioning.
Additionally, the following tests are used to diagnose Klinefelter syndrome, per the Mayo Clinic:
Chromosomal Analysis Also called karyotype, this involves drawing a small sample of blood, which is sent to a lab to check the shape and number of chromosomes.
Hormone Testing This can be done through blood or urine samples and can detect if abnormal sex hormone levels — a sign of Klinefelter syndrome — are present.
Klinefelter Syndrome Treatment
While there is no cure for sex chromosome changes caused by Klinefelter syndrome, treatments can help minimize effects, especially when they’re started early. Treatment may include the following:
Testosterone replacement therapy Since males with Klinefelter syndrome don’t produce enough testosterone, undergoing testosterone replacement therapy at the onset of puberty can allow a boy to undergo the body changes that normally occur at puberty, such as developing a deeper voice, growing facial and body hair, and increasing muscle mass and penis size.
Testosterone therapy can also improve bone density and reduce the risk of fractures, but it will not enlarge the testicles or help with infertility.
Testosterone may be given as an injection or with a gel or patch on the skin.
Fertility Treatments Intracytoplasmic sperm injection (ICSI) can help men with minimal sperm production by removing sperm from the testicle and injecting it directly into a woman’s egg.
Breast Tissue Removal For those who develop enlarged breasts, excess breast tissue can be removed with plastic surgery.
Educational Support Teachers, school counselors, or school nurses may be able to help with finding extra support for learning.
Speech and Physical Therapy These can help overcome problems with speech, language, and muscle weakness.
Psychological Counseling Whether from a family therapist, counselor, or psychologist, counseling can help people with Klinefelter syndrome work through emotional issues related to puberty, young adulthood, and infertility.

Anna C.E. Hurst, MD, MS, FACMG
Medical Reviewer
Dr. Hurst is a physician for the UAB Undiagnosed Disease program, Turner syndrome clinic, and general genetics clinic, and she provides hospital consultations for inpatients at UAB and Children’s of Alabama for general genetics and inborn errors of metabolism. She also is the medical geneticist for the Smith Family Clinic for Genomic Medicine in Huntsville, Alabama.
Hurst's research focuses on expanding the availability of genomic sequencing for children with complex healthcare needs. She also has an interest in how the patient’s physical exam and facial features can be clues to a rare disease diagnosis (dysmorphology) and serves on the scientific advisory board of Facial Dysmorphology Novel Analysis. She has published over 45 peer-reviewed articles in the field of medical genetics, largely focused on the clinical delineation of rare disease phenotypes. She also serves as an associate editor for the American Journal of Medical Genetics.
Hurst is also passionate about education and serves as the program director of the UAB genetics residency programs (categorical, pediatrics-genetics, and internal medicine-genetics) and medical director of the UAB Genetic Counseling Training program. She is an officer with the Association of Professors in Human Medical Genetics.

Cathy Cassata
Author
Cathy Cassata is a freelance writer who specializes in stories about health, mental health, medical news, and inspirational people. She writes with empathy and accuracy, and has a knack for connecting with readers in an insightful and engaging way. Cassata contributes to Healthline, Verywell, Yahoo, and more.
She previously worked for the American Association of Medical Assistants for eight years, writing and editing the magazine, marketing materials, and the website. Cassata completed the editing certificate program at the University of Chicago.
- Klinefelter Syndrome. MedlinePlus. July 10, 2023.
- Klinefelter Syndrome: Symptoms and Causes. Mayo Clinic. September 21, 2019.
- About Klinefelter Syndrome. National Human Genome Research Institute. May 19, 2019.
- About 47,XXY (Klinefelter Syndrome). Association for X and Y Chromosome Variations.
- About 48,XXY. Association for X and Y Chromosome Variations.
- Klinefelter Syndrome: Diagnosis and Treatment. Mayo Clinic. September 21, 2019.