In an attempt to prevent in utero virilization of female fetuses with 21-hydroxylase deficiency, six mothers at risk were treated with either hydrocortisone (n = 1) or dexamethasone (n = 5) in early pregnancy. Treatment was continued to term in the two pregnancies in which the diagnosis of an affected female fetus was confirmed. In patient 1 (hydrocortisone treatment) fetal adrenal suppression was only partial but the external genitalia were only slightly abnormal. In patient 2 (dexamethasone treatment) fetal adrenal suppression was achieved and the external genitalia were normal at birth. These encouraging results open a new prospect for treating congenital adrenal hyperplasia in utero.