Intrahepatic cholestasis of pregnancy (ICP) is a pregnancy-specific liver disease associated with poor maternal and fetal outcome. The diagnosis is based on pruritus with abnormal liver function in the absence of other pathological conditions. However, pruritus in pregnancy is common, and it may be the only presenting feature in ICP. No reliable test currently exists that can discriminate between those women destined to develop ICP and those with the benign condition of pruritus gravidarum (PG). The purpose of this prospective study was to investigate longitudinally the serum concentration of glutathione S-transferase alpha (GSTA, a specific marker of hepatocellular integrity) and to compare this with the temporal profile of conventional liver function markers in women with ICP (n = 63), PG (n = 43), and normal pregnant controls (n = 26). Blood was sampled on at least 3 separate occasions between 16 weeks of gestation and 4 weeks postpartum. Serum concentrations of GSTA increased with gestation in ICP, being significantly higher from 24 (±2) weeks compared with controls (400% difference; 95% CI, 240%–734%; P < .001). GSTA was also higher in ICP versus PG (433% difference; 95% CI, 228%-790%; P < .001) throughout the gestational period studied. Significant differences in the ICP compared with control and PG groups were also found for total bile acids, alanine aminotransferase, aspartate aminotransferase, γ-glutamyl transpeptidase and alkaline phosphatase. In conclusion, the measurement of GSTA provides a test of liver dysfunction that distinguishes women with ICP from those with PG. Additionally, on the basis of this study, reference ranges for biochemical markers of liver function require reevaluation in pregnancy. (HEPATOLOGY 2004;40:1406–1414.)