The LHRH-secreting hypothalamic hamartoma (HH), a congenital malformation consisting of a heterotopic mass of nervous tissue that contains LHRH neurosecretory neurons attached to the tuber cinereum or the floor of the third ventricle, can cause true or central precocious puberty (TPP). We have suggested that it functions as an ectopic LHRH pulse generator independent of the central nervous system inhibitory mechanism that normally restrains the hypothalamic LHRH pulse generator. TPP associated with a hamartoma has all of the hormonal hallmarks of puberty, including a pubertal pattern of pulsatile LH and a pubertal plasma LH response to LHRH administration. Little is known about the natural history of HH. We present long term data on 10 children (5 females and 5 males) with TPP due to HH. Physical signs of puberty were observed at a mean age of 2.2 +/- 1.6 yr (range, 0.5-5.1). Two of 10 had a pedunculated mass, and 8 of 10 had a sessile mass. The hamartoma varied in diameter from 4-25 mm and did not change with time (3.5-8.7 yr). Four patients have a seizure disorder, 3 with gelastic seizures (1 with mental retardation) and 1 with tonic-clonic seizures. The shape of the hamartoma, sessile or pedunculated, did not correlate with the occurrence of seizures. At presentation with sexual precocity, the mean height SD for chronological age was +3.5 +/- 0.4, the mean height SD for bone age was -1.9 +/- 0.4, and the mean bone age SD for chronological age was +6.8 +/- 0.7. Baseline data were comparable to those of 10 females with idiopathic TPP. Nine of 10 HH patients and all idiopathic TPP patients were treated with a LHRH agonist. The response to therapy was excellent in both groups and indistinguishable. Nine of 10 HH children attend school regularly and, aside from those with seizures, have no neurological handicap. While surgical resection of the hamartoma has been recommended, it carries an increased risk of morbidity and mortality and, if removal is incomplete, does not arrest the sexual precocity. In our experience, LHRH agonist therapy for TPP due to HH is the preferable approach.