Two primary bile acids, cholic acid (CA) and chenodeoxycholic acid (CDCA), and one secondary bile acid, deoxycholic acid (DCA), were measured by radioimmuno-assay in pregnancy serum from 30 healthy women, 49 women with itching and 45 with intrahepatic cholestasis of pregnancy. All subjects were studied serially from between 16 and 20 weeks gestation until 35–60 days post partum. In healthy pregnant women, serum CA and DCA levels did not change significantly at any time. The mean CDCA level rose significantly towards term. In women with intrahepatic cholestasis, serum levels of CA and CDCA were increased ten- and five-fold, respectively, at the time of appearance of clinical Symptoms and the CA/CDCA ratio rose from l/l to 2/1; there was also a moderate increase in the serum concentration of DCA. In 4 of 8 women studied prospectively an increase in serum bile acid levels preceded the appearance of Symptoms or other laboratory evidence of intrahepatic cholestasis. Nine of the women with itching with normal routine liver function test results had increases in serum CA and CDCA concen-trations suggesting mild cholestasis.