An acute thrombocytopenic purpura developed shortly after measles-mumps-rubella vaccination in 23 of approximately 700 000 children immunized over a period of seven years. The mean interval from inoculation to the onset of purpura was 19 days. Bone marrow aspirates obtained from 13 patients showed increased or normal amounts of megakaryocytes. Platelet survival time was markedly shortened in the two patients studied. Fifteen patients recovered (the platelet count exceeded 100 x 109/1) in one month, five in two months and two in six months. Increase in platelet-associated immunoglobulin was detected in 10 of 15 patients. Circulating antiplatelet autoantibodies (AAb) against glycoprotein IIb/IIIa were detected in 5 of 15 patients. The findings are compatible with an autoimmune mechanism triggered by immune response to measles-mumps-rubella vaccination. As evaluated by the clinical course and the presence of AAb, post-vaccination thrombocytopenic purpura appears to be indistinguishable from childhood acute idiopathic thrombocytopenic purpura.