Eighty-four patients with neonatal alloimmune thrombocytopenia (NAT) were investigated clinically and by biological laboratory methods. The condition appeared at birth, usually as an isolated thrombocytopenic purpura, but in about 20% of the neonates the haemorrhagic syndrome was associated with signs of infection or with jaundice and hepatosplenomegaly. Considerable variations were observed in the severity of the purpura; in 3 cases the thrombocytopenia was clinically silent. Haemorrhagic brain lesions were present in 7% of the neonates, and severe neurological sequelae in 14 of the 59 children on long-term follow-up. The overall mortality rate was 9.2%. The PLA1 system was involved in 56 of the 59 families studied, with PLA1-negative mothers developing immunization against the foetus' PLA1 antigen. In 20% of these mothers the antibody was not demonstrable, and the diagnosis relied on the mother's phenotype and on a history of previous NAT. The strong association demonstrated between the HLA-DR3 antigen and the ability to develop anti-PLA1 antibodies is of extreme importance. It may be helpful to confirm the diagnosis in mothers without detectable anti-PLA1 antibodies and to identify mothers at risk of alloimmunization. Neurological sequelae, which were due to post-natal haemorrhage in at least 70% of the cases, could now be avoided by an early diagnosis, modern transfusional techniques and caesarian section. However, antenatal lesions cannot be avoided, except by preventive measures, yet to be developed, against alloimmunization or the cytopenic effect of the platelet antibody.