Bile acid concentration was measured in amniotic fluid obtained for standard indications from 11 healthy pregnant women without polyhydramnios (28 to 42 weeks of gestation) and from 9 patients with polyhydramnios (28 to 38 weeks of gestation). Two of the latter women delivered infants with intestinal obstruction distal to the papilla of Vater, a condition that causes regurgitation of bile into the amniotic fluid. In the women without polyhydramnios, the total bile acid concentration ranged from 1.4 to 2.4 µmol/liter. In the seven patients with polyhydramnios not associated with fetal intestinal obstruction, the bile acid concentration in amniotic fluid was not significantly different (0.9 to 1.9 µmol/liter). By contrast, the bile acid concentration in amniotic fluid specimens from the two patients with polyhydramnios who gave birth to children with intestinal obstruction was considerably elevated (30.3 and 83.1 µmol/liter). These findings suggest that determination of bile acid concentration in amniotic fluid permits prenatal diagnosis of intestinal obstruction distal to the papilla of Vater.