Maternal cholestasis affects about 1% of pregnancies in Finland. Although maternal prognosis in obstetric cholestasis is always good, an increased fetal risk has been reported by several authors. In this paper the differential index (DI), describing the short-term variability of fetal heart rate, was measured in 64 pregnancies with colestasis of pregnancy by a microprocessor-based "on-line" method, which uses abdominal fetal electrocardiogram as a triggering signal. The analysis was successfull in 117 of 131 trials. In five pregnancies no successful analysis was obtained. Fetal distress developed in five fetuses of 59 but not perinatal deaths occurred. The sensitivity of the antepartum DI in predicting fetal distress in labor was 80% and the predictive value was 44%. The relative risk for intrapartum fetal distress in labor after a pathologic antepartal DI compared with normal DI was 22, which is highly significant (p less than 0.001).