Fetomaternal bleeding in pregnancy is the most common cause of rhesus immunization. In this study we evaluated the amount of fetomaternal bleeding during pregnancies with and without complications. The data from 1204 patients are analyzed. Fetomaternal bleeding was of clinical relevance (HbF greater than 0.01%) in 6% of all uncomplicated pregnancies. There was no increased fetomaternal bleeding in pregnancies complicated by gestosis, preliminary labour, placenta praevia, trauma, and diabetes in pregnancy. In cases with premature rupture of the amnion or vaginal bleeding in pregnancy we observed an increased percentage of fetomaternal bleeding into the mother's circulation. Nearly 25% of all patients with hydrops fetalis had clinical relevant fetomaternal bleeding (HbF greater than 0.01%).