Six cases of bileaflet mechanical mitral valve thrombosis were serially assessed by Doppler echocardiography and cinefluoroscopy during thrombolytic therapy with urokinase. Two patients had dual mechanical valve replacement in the aortic and mitral positions simultaneously. Bileaflet thrombosis was diagnosed by 2-dimensional echocardiography in three cases, Doppler echocardiography and cinefluoroscopy in six cases. Thrombolytic therapy using urokinase was successful in five patients (3 cases : UK 1,000,000 unit x 3 hours for 2 days, 2 cases ; UK 1,000,000 unit/24 hours for 5 and 7 days respecitively, 1 case ; UK 1,000,000 unit/hr x 3 hours for 1 day). The other one patients had massive cerebral thromboembolism and subsequently died. These study demonstrated the usefulness of Doppler echocardiography and cinefluoroscopy in diagnosis and serial assessment of thrombolytic therapy in the patients with mechanical mitral valve thrombosis.