Pulmonary embolism is an emergency situation. The earlier therapy begins, the more effective it will be. This, however, asks for immediate diagnosis. At present the most effective method of diagnosing and localizing pulmonary embolism is perfusion scintiscan of the lung. It should be completed by chest radiography to exclude peripheral infiltration and/or central lung cancer. Generally, these two procedures ensure optimal diagnosis. In case of additional disease of the pleura, mediastinum and lung parenchyma, further measures may be necessary: especially in chronic obstructive lung disease ventilation perfusion ratio and outwash of xenon might be helpful. Scintiscan of the lung is suited for early diagnosis as well as for follow-up examinations. Only if thrombectomy is planned, pulmonary angiography should be preferred. Scintiscan of the lung with marked particle suspensions ensures a minimum of complications. There is only one incident in 10,000 examinations. According to Quinn (1964) and Felix (1971) pulmonary scintiscan might be harmful in patients with right heart failure. We made no such observation in 4000 cases.