In 40 pregnancies complicated by maternal diabetes mellitus umbilical venous blood was obtained by cordocentesis within 24 hours of elective delivery at 36 to 40 weeks' gestation. The mean fetal hematocrit was significantly higher and the mean platelet count significantly lower than the corresponding values of our reference ranges. Furthermore, blood gas analysis demonstrated these fetuses to be normoxemic but acidemic. The degree of fetal acidemia was significantly associated with both maternal and fetal blood glucose concentrations. The fetal hematologic indices were significantly related to the maternal glycosylated hemoglobin percentage but not to the degree of fetal acidemia or to the maternal or fetal blood glucose concentration at the time of cordocentesis. Fetal acidemia, polycythemia, and thrombocytopenia may contribute to the increased incidence of late unexplained intrauterine deaths in pregnancies complicated by maternal diabetes mellitus.