Heparins are in widespread use as anticoagulants for the prophylaxis and therapy of thromboembolisms. A dangerous side-effect is heparin-induced thrombocytopenia type II (HIT type II) with the paradox of thromboembolic venous and arterial vascular occlusions. HIT type II is an immunological disease which results in activation of platelets and plasma coagulation. The main symptom is an acute onset of thrombocytopenia with a fall in thrombocytes to less than 50% of the initial value with or without newly arising thromboembolic complications between days 5 and 14 after the start of heparin therapy. Surgery patients are more often affected by subclinical antibody formation as well as by symptomatic HIT type II than clinical patients. In this review we will discuss the difficult diagnosis and the differential diagnosis with special emphasis on postoperative intensive care patients, as well as preventive measures and management on occurrence of HIT type II and associated thrombotic complications.