Gynecomastia is a condition of overdevelopment or enlargement of the breast tissue in men or boys. It is caused by an imbalance of the hormones estrogen and testosterone, which can cause an increase in the amount of breast gland tissue. Gynecomastia can affect one or both breasts, sometimes unevenly, and can cause female-appearing breasts. Pseudogynecomastia refers to an increase in fat but not glandular tissue in male breasts.

Gynecomastia can be caused by a variety of factors, including changes in hormone levels during puberty, certain medications, illegal drugs, and health conditions that affect hormone balance. It is usually a benign (noncancerous) condition.

Diagnosis of gynecomastia involves a physical exam and medical history, as well as tests such as blood tests, urine tests, mammogram, and biopsy. Treatment options include medication to adjust hormone imbalances and surgery to remove excess breast tissue. Losing weight or doing more exercise may help in some cases, but it may not always improve the condition.

Gynecomastia is a common condition that affects about half of all guys going through puberty in one or both breasts. It can also occur in newborn babies and older men.