Idiosyncratic drug-induced thrombocytopenia is a relatively rare and potentially serious side-effect in its moderate and profound forms (platelet count less than 100 x 10(9)L). It may be of central or peripheral mechanism, with consumption or immunological destruction. The main incriminated molecules are heparins, quinidine, sulfonamides and gold salts. Hemorrhagic manifestations are the rule and their severity is related to the level of thrombocytopenia and the involved molecule. Heparin-induced thrombocytopenia is more often associated with thrombotic events. Diagnosis relies on medical history and clinical criteria, which also establish the level of imputability. The role of serological tests is not yet clear but seems to be particularly interesting in difficult diagnostic situations and in heparin-induced thrombocytopenia. The treatment is based on the discontinuation of the incriminated drug and on supportive measures that depend on the severity of the clinical features.