Of 67 leukaemic children transplanted in our BMT unit 3 presented with severe acute respiratory syndrome associated with pulmonary thromboembolism (PTE) as diagnosed by scintiscan and/or angiography in the first month after BMT. Intervention with continuous positive pressure ventilation, urokinase (loading dose, then continuous infusion for 12-18 h) and heparin (continuous infusion for an average of 10 days) has been carried out successfully in two cases. In conclusion, when evaluating patients undergoing BMT and developing early pulmonary complications, PTE must be considered. The pathogenesis of PTE is still difficult to ascertain but urokinase therapy may reduce early morbidity.