A 10-month-old boy with major left atrial thrombus following cardiac surgery was treated with intravenously administered recombinant tissue-type plasminogen activator (rt-PA; Actilyse, Thomae-Behring, Germany). The left atrial thrombus was diagnosed by Doppler echocardiography 8 days after complete correction of a ventricular septal defect. rt-PA therapy was administered over a 10-day period. Significant hemopericardium occurred 50 h after the start of thrombolytic therapy. rt-PA infusion was discontinued for 20 h to insert a pericardial drainage. The initial rt-PA dose was 0.1 mg/kg over 10 min followed by a continuous daily infusion of 1.7 mg/kg together with low-dose heparin. Thrombolytic therapy was restarted 20 h after pericardial drainage was inserted. The daily rt-PA dose was gradually raised to 3 mg/kg (total dose: 18 mg/kg). On day 7 and 8 a clear decrease in P-plasminogen and P-antithrombin occurred, requiring additional fresh frozen plasma and P-antithrombin concentrate substitution. One day later, without further side effects, complete thrombolysis occurred. Although hemopericardium demanded discontinuation of thrombolytic therapy, rt-PA administration, closely monitored by Doppler echocardiography, was continued, leading to complete thrombolysis of the left atrial thrombus in the early postoperative period. We consider the literature dealing with rt-PA thrombolysis in infancy we discuss this case report.