A baby girl born at 31 weeks gestation showed severe haemolytic anaemia and hyperbilirubinaemia which led to exchange transfusion within the first 12 hours of life. There was no blood group incompatibility between mother and child but there was a marked stomatocytosis of the baby's red blood cells. Family history revealed a congenital stomatocytosis in the mother. Biochemical characterization of the defect was performed. Phospholipid analysis of the erythrocyte membrane of mother and child showed an increase in phosphatidylserine with a compensatory decrease in phosphatidylcholine and phosphatidylethanolamine. SDS-electrophoresis showed multiple modifications of the protein pattern with a decrease in band 6, an increased content of band 4.1b, a slight decrease in band 7 and a clear change in the shape of the protein band 3 pattern. The results suggest that the basis of the observed abnormalities is a common defect in protein posttranslational modification, rather than multiple genetic defects in the synthesis of several proteins. Haematologic, biochemical and clinical course of the disease in this preterm infant are discussed.