Severe virilization syndrome is seen in testosterone-producing ovarian or adrenal tumor while symptom in late-onset congenital adrenal hyperplasia (CAH) are usually mild hirsutism and amenorrhea. We determined the serum levels of 17 alpha-hydroxy pregnenolone (17P5) and 17 alpha-hydroxyprogesterone (17P4) 60 min after the intravenous injection of 250 micrograms of ACTH in 10 normal Japanese females with age ranging from 18 to 29 years old. In this test, 1 mg of dexamethasone had been administered orally at 11 p.m. on the previous day to suppress the effect of endogenous ACTH. The ratio of 17P5 to 17P4 was 10.0 +/- 3.6 (mean +/- SD). Five females with hirsutism of no ovarian origin showed normal responses to this rapid ACTH test, suggesting that the incidence of the late-onset CAH is not so high as reported previously.