Urokinase was administrated to 36 patients for the thrombolytic treatment of 63 instances of Bjork-Shiley prosthetic valve thrombosis. Patients were diagnosed either from clinical findings or from the maximum opening angle of the prosthetic valve. By means of administration of urokinase, the maximum opening angle of the prosthetic valve, heart rate and cardiothoracic ratio improved significantly. It was considered that fibrinolytic treatment with urokinase against prosthetic valve thrombosis is more effective in the asymptomatic phase, and for this reason, early detection of abnormal prosthetic valve sounds and periodical measurement of the maximum opening angle of the prosthetic valve are significant.