The distribution of glucose-6-phosphate dehydrogenase (G-6-PD) deficiency in ethnic minorities in the Netherlands was studied in a random sample of 668 healthy pregnant women and 754 healthy full term neonates. The overall prevalence of G-6-PD deficiency was 6.6% in males and 5.2% in females. Highest frequencies were found in sub-Saharan blacks. The hematological data in severe deficient women of African descent suggest slight hemolysis in the first trimester of pregnancy. Mean hemoglobin concentrations in pregnant women and neonates of African descent were lower as compared to hemoglobin concentrations in persons of Asian or Mediterranean origin. G-6-PD deficiency was found to be the only cause of neonatal jaundice in 6% of the non-Caucasian neonates who underwent exchange transfusion for severe neonatal hyperbilirubinaemia. Mean bilirubin in cordblood, however, was not found to be significantly higher in severe deficient neonates of African, Asian and Mediterranean descent.