Congenital adrenal hypoplasia is a rare disorder and one of the possible causes of low maternal plasma and/or urine estriol during pregnancy. We studied 2 cases pre- and postnatally. Dehydroepiandrosteronesulfate (DHAS) and estriol (E<sub>3</sub>) studied longitudinally in maternal plasma were at or below the lower limit of the normal range. Prior to substitution treatment, an ACTH test revealed low plasma levels of aldosterone, cortisol and all their precursors, with no rise after ACTH. ACTH plasma levels were elevated. It is possible to detect congenital adrenal hypoplasia prenatally in families at risk by repeated measurements of DHAS and estriol in maternal plasma during pregnancy.