Serum concentrations of dehydroepiandrosterone, DHA sulfate, estradiol, and prolactin in 20 girls with precocious adrenarche were compared with those of healthy girls of varying age and degrees of breast and sex hair development. Production of adrenal androgens, as reflected by serum DHA and DHA-sulfate concentrations, was significantly increased in PA above that in age-matched control subjects. Surprisingly, in spite of their lack of breast growth, the patients with PA also had serum estradiol levels that were higher than in the prepubertal girls and similar to those found in girls with both breast and pubic hair development. Serum prolactin concentrations in the patients with PA were not increased over those of the age-matched (less than 8 years) prepubertal girls. In the older prepubertal ( greater than 8 years) and early pubertal girls serum prolactin levels were lower. The finding of increased estradiol levels suggests that precocious adrenarche is not a distinct endocrine entity, but merely represents a variant of early adolescence in which estrogen secretion is sufficient to influence adrenal 3beta-hydroxysteroid dehydrogenase activity with a resultant rise in DHA production, but not sufficient to produce clinically apparent breast changes. The data do not support a similar role for prolactin.