The present prospective study indicates that children of mothers with blood group O run a double risk of hyperbilirubinemia requiring treatment as compared to children of mothers of blood group A, and 5-10 times increased risk of needing exchange transfusion. The most frequent cause of need for exchange transfusion was ABO-incompatibility between mother and child. A positive direct antiglobulin reaction in an ABO-incompatible child in need of treatment doubles the risk of exchange transfusion being required. Blood group O in the mother should be considered to be an independent risk factor for the child, and O-pregnant women should be ABO-grouped for this reason.