88 families in which 84 cases of neonatal alloimmune thrombocytopenia (NAT) occurred, were studied. In 84 families, the NAT was the consequence of an incompatibility in the PLA system. Furthermore, the phenotype HLA-DR3 increases greatly the risk of immunisation (RR: 76,5). The importance of the risk of neurological sequellae was shown by the clinical study (about 25% of the surviving neonates). The occurrence of the accident at the first birth of a PLA1 positive child in a sibship was frequent (59%). In addition, the NAT recurred at each birth of a PLA1 positive child with only five exceptions. All of them concern a female neonate and this might be meaningful. Therapeutical data are heterogeneous and difficult to interpret. However, it appears that the prevention of obstetrical traumatism by caesarean section and compatible platelet transfusions are useful. It is too early to evaluate the efficacy of prenatal transfusions of mother's washed platelets. However, in the two cases in which we use them, they gave a good and sustained platelet count increment. The prenatal diagnosis of NAT and the PLA grouping of the foetus has been proposed in three cases and are feasible at 20 weeks of pregnancy.