We recently observed five cases of early thrombus formation in patientsundergoing anticoagulation with subcutaneous heparin following open heartsurgery. The reasons prompting surgery were as follows: one mitral valvereplacement, 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 bysubcutaneous (SC) heparin on postoperative day 1. Acute thrombocytopeniawas observed between the 6th and 11th postoperative day. This wasinterpreted as denoting an idiosyncratic reaction to heparin which wasreplaced by low molecular weight heparin (LMWH) in two cases and byacenocoumarol in the other cases. Massive thrombosis of the aortic valveresulted in the death of one patient. Thrombosis of the left atriumoccurred in three patients (two of whom had a transient ischemic attack(TIA)). One patient had aorto-iliac thrombosis. Successful reoperation wascarried out in four of the five patients. Although heparin-inducedthrombocytopenia and thrombosis [HITT] is a rare complication of heparintherapy, serial platelet count monitoring and in vitro platelet aggregationtests are mandatory in the diagnosis of this syndrome. Discontinuation ofheparin is indicated as soon as the syndrome is recognized and theinstitution of aspirin is recommended if the thromboembolic complicationrequires reoperation and reexposure to heparin.