Advances in technology have made possible the prenatal diagnosis and treatment of female fetuses with classical congenital adrenal hyperplasia due to 21-hydroxylase deficiency. Hormonal measurement of 17-hydroxyprogesterone, androstenedione, testosterone and 21-deoxycortisol and HLA typing and DNA analysis for 21-OH/C4/HLA class I and II genes in chorionic villus cells and amniocytes are utilized for prenatal diagnosis. Maternal dexamethasone administration begun in the first trimester has prevented or ameliorated virilization in approximately three-fourths of infants. Maternal estriol levels appear to be the most accurate measure of fetal adrenal suppression. Maternal side effects are not infrequent and include excess weight gain, edema, glucose intolerance, hypertension and gastrointestinal problems. Severe permanent striae have been reported. Although no complications of prenatal treatment in the treated fetus or child have been reported long-term follow-up with careful neuropsychologic evaluation is not yet available and is necessary to fully evaluate possible long-term side-effects of prenatal dexamethasone treatment.