Gynecomastia is becoming more and more common in outpatient clinics, and it occurs around the time of puberty and in old age. Many people ask about the causes of gynecomastia, so today I have briefly organized the etiology and pathology of gynecomastia. I. Endogenous physiological endocrine changes 1. Temporary mammary gland development in newborns. 2. Elderly people. 3. Obesity. The reduction of male breast development after the age of 70 is related to weight loss. II. Exogenous endocrine changes caused by foreign intake or absorption of estrogen. Third, endogenous pathological endocrine changes 1, testicular tumors, either benign or malignant teratomas, hyperplasia or tumors of the anterior pituitary gland. 2, cirrhosis of the liver as one of the important causes of male breast enlargement. 3, Chronic lung disease with proliferative osteoarthropathy other than bronchial lung cancer. 4, hyperthyroidism or hypoadrenocorticism can sometimes cause mild male breast enlargement.