Five cases of thrombosis of cardiac prosthetic valves were treated with high dose fibrinolytic drugs during a short period of time. There were 3 mitral, one tricuspid and one pulmonary prosthetic valves. In 3 cases the treatment was successful without significant complications. Another patient, a woman in early pregnancy, died shortly after the treatment. The last patient required surgery as the cause of valve obstruction was a fibrous pannus virtually without thrombus formation. This diagnosis was intraoperative. The reported therapeutic modality permits a rapid resolution of valve obstruction. If this is not the case, surgery may be immediately carried out when the hemodynamic condition of the patient is still adequate.