Between October 1991 and January 1995, 10 patients presented 14 episodes of thrombosis of mechanical valve prosthesis, 11 obstructive, 3 nonobstructive. In two cases, the thrombosis was recurrent (one after thrombolysis, one after surgical thrombectomy). In another two cases, the thrombosis was a recurrence (on a valve already surgically replaced because of thrombosis). Anticoagulant therapy had been ineffective in 9 cases; protein S deficiency was diagnosed in one case. Transoesophageal echography allowed diagnoses in all cases. Thrombolysis was the treatment of first intention in 9 cases (completed by a second course of thrombolysis in one case and by valvular replacement in two cases. The other patients were managed by immediate valve replacement in two cases, thrombectomy in one case, long-term parenteral anticoagulation in two cases (one of which was followed by valve replacement). Transoesophageal echography showed improved valve motion. Incomplete thrombus dissolution was observed in 50% of cases. Thrombolytic therapy was complicated by cerebrovascular accidents in two patients, one of which was fatal. One patient had regressive hemiplegia, one patient had a local hematoma. The authors conclude that thrombosis is a serious complication of valve replacement and usually occurs in patients inadequately anticoagulated. Thrombolysis may enable some patients to avoid reoperation but its risks limits its use to those patients thought to be unacceptable surgical risks. An apparently high frequency of this complication durind this period has led to the initiation of a study to determine the predisposing factors and to put preventive measures into action.