Development of mammary glands in a boy or man; glandular tissue can be felt under the nipple. The condition usually occurs in puberty, and can affect one or both breasts. The cause is unknown. The condition can be somewhat painful, but the patient suffers most from embarrassment, because he is often the butt of teasing at swimmmimg or other sports. In younger patients the condition disappears spontaneously after a few years; surgery is resorted to only if the boy is undergoing acute psychological distress. Gynaecomastia can be just as harmless in older patients, but can also be associated with disorders elsewhere in the body, such as tumours of the testicles or the adrenal cortex, which may produce substances that stimulate growth of the mammary glandular tissue. Liver conditions (cirrhosis of the liver), hyperthyroidism or hormonal treatment of cancer of the prostate can also cause gynaecomastia. In older men enlargement of the breast can (rarely) be caused by cancer of the breast.