The importance of the placental 3 beta-steroid sulfatase for placental biosynthesis of estrogens is demonstrated by a case report of a placental sulfatase deficiency. The absolute deficiency of this enzyme in our case is demonstrated in vivo by the intravenous dehydroepiandrosterone sulfate loading test and in vitro by placental enzyme tests. Steroid concentrations in serum after injection of dehydroepiandrosterone sulfate (DHEA-S) are compared with those in a group of pregnant women without placental enzyme defects and in a group of nonpregnant women. Placental in vitro tests demonstrate the intact 3 beta-hydroxysteroid dehydrogenase-delta 4,5 isomerase-aromatase-system in the placenta with sulfatase deficiency. The lack of placental hydrolysis results in low concentrations of estrone and estradiol-17 beta in the maternal serum, which are only 5.9% and 12.5%, respectively, of the mean values of the control group. This indicates that more than 90% of estrone and more than 85% of estradiol-17 beta measured in the maternal serum are from DHEA-S as precursor. The remaining concentrations are converted mainly from dehydroepiandrosterone (DHEA), which needs no hydrolysis. Maternal serum concentrations of estriol are under the detection limit of the radioimmunoassay. This is due to the absence of the so called "neutral pathway" and the "phenolic pathway" with 16 alpha-hydroxydehydroepiandrosterone sulfate (16 alpha-OH-DHEA-S) and DHEA-S respectively as precursors. The insignificance of the "placental pathway" for the total biosynthesis of estriol is demonstrated. The placental sulfatase seems to have no great importance for the biosynthesis of the C21-steroids.