Gynecomastia is often due to an imbalance of testosterone and estrogen hormones. It is an increase in the amount of breast gland tissue in boys or men. Gynecomastia can affect one or both breasts, sometimes unevenly. Genetic disorder Malnutrition Testicular cancer Aging Disorder of testes causes: Genetic disorder Malnutrition Testicular cancer Aging Disorder of testes S/S Pain, particularly in adolescents Swollen breast tissue Breast tenderness Nipple discharge in one or both breasts