Among the causes of precocious puberty, hypothalamic hamartoma comprises a small percentage. However, the frequency of precocious puberty in the presence of hypothalamic hamartoma is quite high. Recently, results of surgery in 14 cases of hypothalamic hamartoma were reported. Precocious puberty completely subsided in three cases and slight improvement was achieved in another three cases. We performed surgery in four patients with hypothalamic hamartomas, with the goal of decreasing the symptoms of precocious puberty. The patients were two females (aged 1 yr, 3 mo and 6 mo) and two males (aged 3 yr, 7 mo and 1 yr, 9 mo). The main symptoms were precocious puberty and mental retardation of varying degrees. The males had excessive growth of body and external genitalia, while the females had genital bleeding and premature breast development. In each case, computed tomographic scans disclosed a round, isodense mass in the interpeduncular cistern, attached to the base of the hypothalamus. Contrast enhancement was negative. Endocrinologically, in case 1, testosterone was 92.6 ng/ml, FSH was 16 mIU/ml, and LH was 2.2 mIU/ml. Although LH was within normal limits, it overresponded to LH-RH stimulation. In case 2, estrogen was 13.5 ng/day, LH was 5.2 mIU/ml, FSH was 5.3 mIU/ml, and LH showed an exaggerated response to LH-RH stimulation. In case 3, testosterone was 362 ng/ml, LH was 8.8 mIU/ml, FSH was 4.8 mIU/ml, and LH showed an abnormally high response to LH-RH stimulation. In case 4, LH was 18.4 mIU/ml, FSH was 12.0 mIU/ml, and both hormones were stimulated abnormally strongly by LH-RH.(ABSTRACT TRUNCATED AT 250 WORDS)