SUMMARY.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 10times1091‐1. 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‐la negative patients (83%) had platelet‐specific HPA‐la antibodies and two HPA‐lapositive individuals had anti‐HPA‐lbantibodies. 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 IgGl and IgG3 subclasses during the thrombocytopenic period while all of 10 patients had anti‐HPA of only the IgGl 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 i