Three baby girls between 22 and 30 months of age, presenting with isosexual idiopathic precocious puberty apparently not due to any organic cause, were studied. Basal levels of plasma steroids of adrenal and gonadal origin, circadian rhythm of plasma cortisol, and pituitary response to 25 microgram of LHRH were evaluated. All cases were characterized by high levels of plasma gonadotropins and by a marked response to exogenous LHRH. Normal cortisol circadia rhythm was found in all cases, one of which characterized by slightly raised plasma values. The other adrenal steroids were all higher than those expected for the chronological age, corresponding to those of 5-6 years old girls. On the other hand, steroids of both adrenal and ovarian (A, T) or mainly ovarian origin (E2) and DHT were all found to be higher than those normally reported in girls at stage 2 of sexual development. These data indicate a hypersecretion of gonadotropins in idiopathic isosexual precocious puberty, with a marked gonadal steroidogenetic response. The secretion of adrenal androgens does not appear to have an important role in the etiology of this condition.