Fibrinogenolysis induced by thrombolytics exposes the risk of haemorrhagic complications. The efficacy is proved for pulmonary emboli of recent origin. The aim of this study is to research into the effect of local administration of low dose urokinase in six patients aged 40 (+/- 16 years) and presenting with old emboli (10 +/- 4 days). The initial clinic picture was serious with shock (2 cases), hypoxaemia (6 cases), pulmonary arterial hypertension (mean 40 +/- 8 mmHg) and a Miller index of 58 (+/- 8%). Mechanical ventilation was necessary four times. Urokinase was administered in situ using a Swan Ganz catheter, with 1,000 units per Kg per hour for six hours followed in sequence with 30 microkatals per hour of plasminogen for two hours. This eight hourly rotating sequence was followed for at least 72 hours. Six patients were cured with an end of treatment (5 +/- 2 days) improvement in their hypoxaemia of 22%, a fall of 47% of the pulmonary arterial pressure and a rise of 71% in the Miller index. The fibrinogenesis fell by 11% and the thrombolytics could not blamed for any side-effect. The sequence urokinase-plasminogen in low dose administered locally may represent an alternative treatment for severe and long standing pulmonary emboli in patients with a risk of haemorrhage.