In patients with congenital adrenal hyperplasia (CAH) due to 21-hydroxylase deficiency, the concentrations of the cortisol precursor 17-alpha hydroxyprogesterone (17-OHP) and its metabolite delta 4-androstenedione (delta 4 A) are increased. CAH was diagnosed in twins by measurement of 17-OHP and delta 4 A concentrations in amniotic fluid obtained by amniocentesis from both amniotic cavities at 17 weeks' gestation. Both prenatal karyotypes were 46,XX. Spontaneous labor and delivery of 2 nonviable fetuses with genital masculinization occurred at 26 weeks' gestation. It is concluded that delta 4 A measurement, like 17-OHP quantitation, is valuable in the prenatal diagnosis of CAH; that both methods appear useful in prediction of CAH in twin fetuses; and that abnormal adrenal-mediated masculinization in female CAH is well established before the end of the second trimester.