Alloimmune thrombocytopenia (AIT) in fetuses and newborn, a disease resembling Rh incompatibility, is caused by transplacental transfer of an IgG-class antibody against fetal platelets. In humans five different platelet antigen systems are so far known which lead to AIT. The disease occurs in 1:2000-1:5000 deliveries. In contrast to Rh disease, immunization occurs in the first pregnancy in the majority of cases. The main significance of AIT lies in the occurrence of fetal (-10%) and neonatal (-20%) intracranial hemorrhage. Newborns are treated with compatible platelets, if necessary in combination with immunoglobulins. The high rate of fetal intracranial hemorrhage justifies therapy during pregnancy as well. Antenatal measures include treatment of the mother with high-dose immunoglobulin, treatment of the fetus with immunoglobulin by cordocentesis, and fetal platelet transfusions. However, all therapeutic measures involving the fetus remain in the experimental stage at present. International cooperative studies are necessary to evaluate cost-benefit of intervention during pregnancy.