The clinical and immunological data of 12 Danish cases of post-transfusion purpura (PTP) are summarized. All patients but one were women. All except one had thrombocytopenic purpura which occurred 4-11 days after transfusion, usually with a nadir of the platelet count below 10 x 10(9) 1(-1). The typical haemorrhagic symptoms were cutaneous bleedings, melena and haematuria lasting from 3 to 12 days. The time until normalization of platelet count varied from 3 to 50 days after onset. One patient had recurrence of PTP and one patient died due to intracranial haemorrhage. Ten of the HPA-1a negative patients (83%) had platelet-specific HPA-1a antibodies and two HPA-1a positive individuals had anti-HPA-1b antibodies. In 10 patients, HLA antibodies were also detectable and in one patient a delayed haemolytic transfusion reaction due to anti-E was seen. All of seven patients investigated had anti-HPA antibodies of both IgG1 and IgG3 subclasses during the thrombocytopenic period while all of 10 patients had anti-HPA of only the IgG1 subclass after recovery from PTP. Thus, the destruction of autologous platelets in PTP seems to be associated with the presence of anti-HPA of the IgG3 subclass which may be of importance in the pathogenesis of PTP.