We recently observed five cases of early thrombus formation in patients undergoing anticoagulation with subcutaneous heparin following open heart surgery. The reasons prompting surgery were as follows: one mitral valve replacement, one double valve replacement, one mitral valve reconstruction, one aortic valve replacement associated with coronary bypass. In all cases, intravenous heparin was begun on the day of surgery and replaced by subcutaneous (SC) heparin on postoperative day 1. Acute thrombocytopenia was observed between the 6th and 11th postoperative day. This was interpreted as denoting an idiosyncratic reaction to heparin which was replaced by low molecular weight heparin (LMWH) in two cases and by acenocoumarol in the other cases. Massive thrombosis of the aortic valve resulted in the death of one patient. Thrombosis of the left atrium occurred in three patients (two of whom had a transient ischemic attack (TIA)). One patient had aorto-iliac thrombosis. Successful reoperation was carried out in four of the five patients. Although heparin-induced thrombocytopenia and thrombosis [HITT] is a rare complication of heparin therapy, serial platelet count monitoring and in vitro platelet aggregation tests are mandatory in the diagnosis of this syndrome. Discontinuation of heparin is indicated as soon as the syndrome is recognized and the institution of aspirin is recommended if the thromboembolic complication requires reoperation and reexposure to heparin.