Among the immediate transfusion reactions caused by the utilization of blood products, those suggesting immuno-allergic mechanisms posed problems for frequency, gravity, laboratory diagnosis and safety. We report here the Lille Hospital's experience over a four-year period concerning these manifestations after platelet concentrate transfusion. Eight hundred and fifty-two immediate transfusion reactions have been declared, of which 230 were allergic, which appeared in 181 patients (27%). Among the most frequent clinical signs, rash was often described (158 cases: 68.7%); less frequent were respiratory problems such as dyspnea (34 cases: 14.8%) and hypotensive reactions (18 cases: 7.8%). Seven patients presented severe reactions (3%). Twenty percent of them presented multiple allergic reactions and in 43.2%, the recurrence was more serious than the initial problem in spite of preventive medication; the use of washed blood components was necessary. The age of platelet concentrates does not appear to play a part in provoking these events (67% of platelet concentrates had been collected within four days). These allergic transfusion reactions posed problems for those who prescribe medication, because they are frequent, sometimes serious, can recur and at present, the proposed medication prevention is not always efficient.