Thrombolysis in massive embolism is associated with a number of complications, mainly haemorrhagic, justifying an evaluation of the individual risk/benefit ratio before prescription. Major bleeding episodes, the most serious complication, are observed in 8 to 15% of cases and result from mechanisms involving the coagulation factors, platelet function and the vessel wall. In over 70% of cases, they are directly related to sites of vascular puncture, especially for the carrying out of pulmonary angiography which triples the haemorrhagic risk. Uncontrolled hypertension, the prescription of active oral anticoagulants at the time of thrombolysis, low body weight, diabetes, female gender, and ages over 70 have also been identified as factors predictive of bleeding. The prevention of haemorrhage is based on strict observance of the contraindications of thrombolysis and the limitation of early vascular puncture sites, especially femoral, for pulmonary angiography. In cases of haemorrhage, treatment requires interruption of thrombolysis and heparin, the neutralisation of circulating plasmin, inactivation of plasminogen and correction of fibrinogen-induced deficits. Allergic reaction may also occur during thrombolysis, especially with the use of streptokinase. They are usually mild and be prevented by prior routine administration of steroids. The same applies to hypotensive episodes, also more common with streptokinase and rapidly reversible by transient interruption of thrombolysis.