Hello, Gynecomastia is swelling of the breast tissue in boys or men, caused by an imbalance of the hormones estrogen and testosterone. Gynecomastia can affect one or both breasts, sometimes unevenly. Most cases of gynecomastia regress over time without treatment. If gynecomastia is caused by an underlying condition, such as hypogonadism, malnutrition or cirrhosis, that condition may need treatment.