Alloimmunization against platelet glycoprotein IIb and/or IIIa is a complication rarely observed during the evolution of type I Glanzmann's thrombasthenic patients. The occurrence of such alloantibodies is usually due to repeated blood transfusion and greatly complicates the treatment of these patients since they prevent effective platelet transfusion and might, theoretically, cause posttransfusion purpura. We describe the case of a newborn thrombasthenic patient who developed an IgG platelet allo-antibody 1 month after birth. The diagnosis of Glanzmann's thrombasthenia was complicated by the rare platelet phenotype (PLA1-negative PLA2-positive) of the healthy mother, which was probably heterozygous for the abnormal thrombasthenic gene. Immunofluorescence and immunoblotting techniques demonstrated that the patient antibody was principally directed against the platelet glycoprotein IIIa. Surprisingly, this patient had only received four blood transfusions (fresh frozen plasma on days 1 and 2, and standard red blood cell concentrates on days 5 and 6) before the discovery of the antibody, suggesting prior in utero sensitization. This study emphasizes the need for early diagnosis of the disease. Thrombasthenic patients should be transfused with deleukocyted platelet-free blood products.