Seventy women with rhesus isoimmunization, referred to hospital in Newcastle upon Tyne, when compared with the overall hospital population, showed an excess of pregnancies by more than one partner. By comparing them with a control group it was demonstrated that the differences in marital pattern were explained in terms of age, parity and social class. One or other parent of babies affected by haemolytic disease of the newborn was more likely than controls to undergo subsequent sterilization.