Erythrocyte aggregation was studied in 60 samples of cord blood taken at delivery from neonates whose mothers suffered from hypertension or diabetes in pregnancy or from neonates who showed hypoxia during labor. Eryhtrocyte aggregation significantly increased in the cord blood of neonates who suffered from hypoxia during labor, and in neonates from hypertensive and diabetic mothers. The effects of maternal disease or intrapartum hypoxia may therefore stimulate fetal erythropoiesis and cause major changes in cord blood rheology.