Placental steroid sulfatase deficiency is an unusual cause of low estriol production during pregnancy. Its importance lies in the differentiation of this disorder from the more ominous fetal defects that result in low estriol levels. Serum free estriol levels were found to be low or absent in a 25-year-old gravida 3, para 2 woman, while placental lactogen and chorionic gonadotropin levels were normal. An abdominal x-ray revealed no apparent congenital abnormalities and an oxytocin challenge test was negative. The dehydroepiandrosterone sulfate (DHEA-S) level in the patient's amniotic fluid was 6.8 to 18.4 times greater than those found in control amniotic fluids. The patient's amniotic fluid cortisol level was normal. Twenty-four hours following a normal, spontaneous labor and delivery at 39 weeks, the male infant underwent a synthetic ACTH1-24 stimulation test, with serum cortisols rising from 3.7 to 46 mug/dl at 1 hour. The placenta was morphologically normal on gross, light, and electron microscopic examinations. Steroid 3-alcohol sulfatase and arylsulfatase activities in the patient's placenta were virtually absent. These data indicate that this benign cause of low serum estriol levels may be diagnosed prenatally by elevated amniotic fluid DHEA-S levels.