21-Hydroxylase deficiency results in congenital adrenal hyperplasia and leads to masculinization of the external genitalia of affected females. This complication could be avoided if fetal adrenal gland function were suppressed. A woman with mild 21-hydroxylase deficiency whose previous female child had classic congenital adrenal hyperplasia with masculinization was given dexamethasone beginning at the tenth week of gestation. Maternal estriol and cortisol values indicated rapid and sustained fetal and maternal adrenal gland suppression. At 39 weeks' gestation, the patient was spontaneously delivered of a female neonate with normal external genitalia. Postnatal tests indicated the infant was a single heterozygote for 21-hydroxylase deficiency. This study demonstrates prolonged suppression of the fetal adrenal gland with dexamethasone and suggests it might prevent abnormal masculinization in fetuses with severe congenital adrenal hyperplasia.