- Klinefelter's syndrome
- XXY syndrome
Because XXY males do not really appear different from other males and because they may not have any or mild symptoms, XXY males often don't know they have Klinefelter syndrome (KS). Whether or not a male with KS has visible symptoms depends on many factors, including how much testosterone his body makes, if he is mosaic (with both XY and XXY cells), and his age when the condition is diagnosed and treated. Symptoms may include:
- Abnormal body proportions (long legs, short trunk, narrower shoulders and wider hips)
- Breast growth (gynecomastia)
- Infertility: Nearly all men with KS are unable to father a biologically-related child without help from a fertility specialist.
- Decreased sexual interest
- Less than normal amount of pubic, armpit, and facial hair
- Small, firm testicles
- Tall height
Boys with Klinefelter syndrome may also have learning disabilities and difficulty with speech and language development. They tend to be quiet, sensitive, and unassertive, but personality characteristics vary among males with this condition. 
The Human Phenotype Ontology provides the following list of signs and symptoms for Klinefelter syndrome. If the information is available, the table below includes how often the symptom is seen in people with this condition. You can use the MedlinePlus Medical Dictionary to look up the definitions for these medical terms.
The Human Phenotype Ontology (HPO) has collected information on how often a sign or symptom occurs in a condition. Much of this information comes from Orphanet, a European rare disease database. The frequency of a sign or symptom is usually listed as a rough estimate of the percentage of patients who have that feature.
The frequency may also be listed as a fraction. The first number of the fraction is how many people had the symptom, and the second number is the total number of people who were examined in one study. For example, a frequency of 25/25 means that in a study of 25 people all patients were found to have that symptom. Because these frequencies are based on a specific study, the fractions may be different if another group of patients are examined.
Sometimes, no information on frequency is available. In these cases, the sign or symptom may be rare or common.
- What are common symptoms of Klinefelter syndrome (KS)?. Eunice Kennedy Shriver National Institute of Child Health and Human Development. http://www.nichd.nih.gov/health/topics/klinefelter/conditioninfo/Pages/symptoms.aspx. Accessed 4/21/2015.
- Kirmse B. Klinefelter syndrome. MedlinePlus Web site. December 11, 2006; http://www.nlm.nih.gov/medlineplus/ency/article/000382.htm. Accessed 4/1/2008.