Three boys 3.3 to 3.9 years old, who had precocious puberty that was unresponsive to an analogue of gonadotropin-releasing hormone, were treated with the antifungal agent ketoconazole for up to 12 months. Within 48 hours the serum testosterone concentration fell to normal in two boys and was significantly reduced in the third, paralleling major improvements in behavior. Reciprocal changes in serum levels of 17-hydroxyprogesterone suggested that C17-20 lyase was the principal site of drug action. Although there was evidence of a blunted cortisol reserve during the first week of treatment, the cortisol response to ACTH1-24 had returned to normal by one month of continuous treatment, and normal diurnal cortisol rhythm was preserved. No adverse clinical or biochemical side effects were noted during 9 to 12 months of continuing treatment. During that time, growth velocity was significantly reduced in all three boys, from a mean rate of 1.5 +/- 2.0 cm per year before treatment to 5.9 +/- 0.6 cm per year after ketoconazole therapy. There was a simultaneous retardation of the rate of skeletal maturation. The striking improvements in behavior were sustained for the duration of treatment. These preliminary data suggest that administration of ketoconazole may be a satisfactory treatment for precocious puberty in boys and possibly for other conditions characterized by androgen excess.