Forty amniotic fluid samples of both sexes were collected in the 16th gestational week and analysed for 17 alpha-hydroxyprogesterone (17-OHP), cortisol, and aldosterone. Hormone concentrations were also analyzed in amniotic fluid of a male fetus postnatally diagnosed as having the salt-losing form of congenital adrenal hyperplasia (CAH). The mother also had CAH of the non-salt-losing type. Hormone concentrations were also determined prenatally in two pregnancies at risk. Amniotic fluid from the pregnancy with the CAH fetus had an approx. six-fold elevated 17 alpha-hydroxyprogester one level, whereas cortisol and aldosterone levels were within the normal range when compared with controls. Hormone concentrations in the two pregnancies at risk were within normal limits and subsequently a healthy boy and a healthy girl were born. Institution of cortisone in the mother during early pregnancy was made in one of the pregnancies. The treatment was withdrawn when normal amniotic hormonal levels were obtained. No sex difference in concentrations of 17-OHP or cortisol was found in the controls, whereas there was a marked sex difference in aldosterone levels (p less than 0.005) with male fetuses having higher concentrations. Diagnosis of CAH in the 16th gestational week can be made by analysing amniotic fluid concentrations of 17 OHP. As cortisol and aldosterone levels were within normal range of the CAH-fetus, they are probably of little use for the prenatal diagnosis of CAH-fetuses/or to differentiate the simple virilizing and the salt-losing forms of CAH. Restriction fragment length polymorphism (RFLP)- analyses of the 21-hydroxylase gene in the fetus gave little further information.