A pregnancy has been investigated in which maternal urinary excretion of estrogens was abnormally low. The excretion of estrone, estradiol and estriol was, respectively, 15%, 15% and 5% of levels found in normal pregnancy. Urinary pregnanediol excretion was within the normal range. There was no evidence of either fetal abnormality or growth retardation. A normal male baby was delivered by cesarian section at 39 weeks gestation. Both in vivo and in vitro the placenta was shown to have a normal potential for metabolism of 3β-hydroxy-androst-5-en- 17-one (DHEA) to estrogens, but lacked ability to hydrolyze androst-5-en-17-one-3β-yl sulfate (DHEAS). In vitro incubation experiments with the substrates pregnenolone-7-3H sulfate, estrone- 4-14C sulfate and nitrocatechol sulfate indicated that the placenta was also deficient in sulfatase activity with respect to these substrates. Cord plasma levels of l6α-hydroxyandrost-5-en-17-one- 3β-yl sulfate (160HDHEAS) and pregn-5-en-2O-one-3β-yl sulfate (PS) were in the range for normal pregnancy while the DHEAS level was slightly below normal. We conclude that the cause of the low production of estrogen was deficiency in placental sulfatase activity.