Gynaecomastia (enlargement of the male breast tissue) is a common finding in the general population. Most cases of gynaecomastia are benign and of cosmetic, rather than clinical, importance. However, the condition might cause local pain and tenderness, could occasionally be the result of a serious underlying illness or a medication, or be inherited. Breast cancer in men is much less common than benign gynaecomastia, and the two conditions can usually be distinguished by a careful physical examination. Estrogens are known to stimulate the growth of breast tissue, whereas androgens inhibit it; most cases of gynaecomastia result from deficient androgen action or excessive estrogen action in the breast tissue. In some cases, such as pubertal gynaecomastia, the breast enlargement resolves spontaneously. In other situations, more active treatment might be required to correct an underlying condition (such as hyperthyroidism or a benign Leydig cell tumour of the testis) or medications that could cause breast enlargement (such as spironolactone) might need to be discontinued. For men with hypogonadism, administration of androgens might be helpful, as might antiestrogen therapy in men with endogenous overproduction of estrogens. Surgery to remove the enlarged breast tissue might be necessary when gynaecomastia does not resolve spontaneously or with medical therapy.