['Non-obstructive prosthetic valve thrombosis (PVT) is a unique subset that features clinical presentation without heart failure, and may be asymptomatic. Thrombolysis has been accepted for obstructive PVT, but treatment strategies of non-obstructive PVT are controversial. This study compared the efficacy and safety of thrombolysis and heparin treatment in these patients.', 'Between 1993 and 1998, 20 consecutive patients were found by multiplane transesophageal echocardiography (TEE) to have non-obstructive PVT. TEE was performed for peripheral embolism in two patients, stroke or transient ischemic attack in six, stroke and fever in two, fever in one patient, as a routine postoperative examination in two patients, and for other reasons in seven. Patients were allocated to two groups: group I (n = 8) received streptokinase-mediated fibrinolysis; group II (n = 12) received intravenous heparin by infusion. Treatment was monitored using TEE.', "There was no difference between patient groups with regard to sex, age, type of prosthesis and time since operation, though anticoagulant status was more often inadequate in group II. By TEE, valve motion was normal in all patients. In group I, all thrombi were mobile and 5-13 mm in diameter; in group II, all thrombi but three were mobile and 3-18 mm in diameter. In group I, thrombolysis was successful in all patients, without complications, within 6-72 h. In group II, heparin treatment was successful in six patients in 3-32 days. In one patient, seven days' of unsuccessful heparin was followed by two months' successful coumarin therapy. Among five unsuccessful cases, the thrombus size increased in four (three became obstructive in 7-35 days); all four patients were switched to fibrinolysis, which was successful without complications in 12-60 h. The fifth patient developed a stroke after nine days of heparin treatment and was subsequently operated on."] Is thrombolysis superior to heparin for non-obstructive mitral mechanical valve thrombosis?