Forty one patients were admitted with a diagnosis of prosthetic valve thrombosis. One patient was thrombolysed successfully, and 40 underwent surgical procedures. In 13 cases, prosthetic valve thrombosis occurred in the first month after valve replacement. 18 patients were in functional class IV of the NYHA classification. Among the thrombosed prostheses, 91% were mechanical, and 61% in mitral position. Adequacy of anti-coagulation was the most important risk factor, as this treatment was inappropriate in 20 patients. In 10 other patients, it had been changed for medical (bleeding events, pregnancy) or surgical (non cardiac surgery) reasons. The perioperative mortality rate was high (32.4%). It was 12% in patients in functional class II and 46% in those in class IV (p < 0.05). This underlines the value of early diagnosis, in which echocardiography, especially with a transesophageal probe, takes a major place, allowing prompt surgical treatment.