Abstract: Objective To investigate the clinical characteristics of precocity in boys. Methods Retrospectively analysis of 26 cases with precocity in boys in Peking Union Medical College Hospital from 1988 to 2005. Congenital adrenal hyperplasia (CAH), germinoma and hypothalamic hamartoma are the main causes of precocity. Clinical characteristics and hormone levels were compared among three groups. Results In 26 cases, 8 were CAH, 7 were germinoma, 5 were hypothalamic hamartoma, 3 were idiopathic precocity, 2 were adrenal adenoma, 1 was due to the complication with encephalitis. The levels of serum luteinizing hormone(LH)，follicular stimulating hormone(FSH)，total testosterone(T)，testis volume，and duration of manifestation between three groups were significantly different. Conclusion CAH, germinoma and hypothalamic hamartoma were the main causes of precocity in boys. Early diagnose is essentially important for further treatment and prognosis.