Three children with major intracardiac or vascular thrombosis following cardiac operations were treated with intravenous recombinant tissue-type plasminogen activator. The first patient, aged 10 yr, developed a left atrial thrombus following replacement of the mitral valve with a Björk-Shiley prosthesis. The second patient, aged 16 months, had a right atrial thrombus following a modified Fontan procedure for tricuspid atresia. Both were successfully treated with a short course of intravenous tissue plasminogen activator. The third patient, aged 19 months, developed the Budd-Chiari syndrome with occlusion of the inferior caval vein following a modified Fontan operation for double inlet left ventricle. Even though near-complete thrombolysis and recanalization of the inferior caval vein was achieved with three courses of tissue plasminogen activator on successive days, she died with failure of multiple organs. In all cases, the diagnosis was established by cross-sectional and Doppler echocardiography, and the response to therapy was monitored using the same technique. Thrombolytic therapy with systemic tissue-type plasminogen activator was safe and effective in the postoperative period, with no major haemorrhagic complications.