Background: In this study, we investigated the clinical importance and possibility of mean platelet volume to be a predictive marker for acute mechanical valve thrombosis. Methods: Mean platelet volume was measured in 28 consecutive patients (mean age 47.3±12.8 years) with mechanical valve thrombosis and 91 control subjects (mean age 46.1±5.8 years). In the study group, blood samples were collected at the first admission (group 1, first measurement) and in the postoperative routine control after discharge (group 1, second measurement). The first control group (group 2) was defined by having a mechanical valve with no history of thrombosis of the mechanical valve. The second control group (group 3) was selected in a consecutive manner from the catheterized cardiac patients who proved to have normal coronary angiograms. Diagnosis of mechanical valve obstruction was based on clinical examination, echocardiography, and cinefluoroscopy. Results: Mean platelet volume was significantly higher in group 1 (first measurement) patients than in group 1 (second measurement), group 2 and group 3 patients (p<0.05). There were slightly higher mean platelet volume values in group 1 (second measurement) than in group 2 and group 3, although they were not statistical significance (p>0.05). Conclusion: It was shown for the first time that patients with mechanical valve thrombosis have higher mean platelet volumes than control subjects. Therefore, mean platelet volume might be used as a follow-up marker of thrombosis in patients with mechanical valve thrombosis.