Twenty-seven patients underwent reoperation because of thrombotic obstruction of a disc-type cardiac valve prosthesis. Preoperative clinical features included effort dyspnea in 81%, new-onset angina in 44%, a new murmur in 89%, and an abnormal opening or closing sound associated with the prosthetic valve in 56%. Symptoms were present for 1 week or more before reoperation in 86%, although many patients were referred only after acute exacerbation of heart failure and development of pulmonary edema. Noninvasive studies confirmed prosthetic valve malfunction in only 33%, but cardiac catheterization documented thrombotic obstruction in all 15 patients in whom it was performed. In 14 of the 27 patients, prothrombin time was in the therapeutic range at the time of admission. Prompt reoperation for valve replacement or thrombectomy was performed with an operative mortality of 11%, and long-term outcome was satisfactory in all but 1 hospital survivor. These findings emphasize the importance of considering the diagnosis of thrombosed heart valves in patients with mechanical heart valves who are seen with nonspecific symptoms.