Abstract : Heparin-induced thrombopenia (HIT) may occur as a complication of any type of heparin but is more frequently observed after administration of unfractioned heparins. Such a complication is scarce but potentially severe and may cause life-threatening thromboembolic and hemorrhagic disorders. Tow types of HIT are described. Type I HIT are of non immunologic origin. They have no clinical expression and spontaneously resolve after heparin therapy has been stopped. Type II HIT which are potentially severe and of immunologic origin, are related to the formation of a IgG-platelet factor 4 complex which activates platelets. They are characterized by an absolute or relative reduction of the platelet number observed on two successive assays. Once other causes of thrombopenia have been ruled out, the diagnostic relies on immunologic or functional tests evidencing the platelet anti-factor 4. Stopping heparin administration is mandatory and surrogate anti-thombotic measures must be instituted. An early diagnosis and an appropriate treatment of this complication result in a significant reduction of morbidity and mortality.