Pulmonary embolism is a complication of venous thrombosis. It is a major cause of morbidity and mortality, but a condition that is highly preventable and, when diagnosed, very effectively treated. Yet in the United States, more people die of pulmonary embolism each year than of AIDS. The clinician must maintain a high index of suspicion at all times in the patient at risk for this entity, keeping in mind that overall only about one-third of the patients suspected of having pulmonary embolism actually turn out to have it. A strong suspicion alone is not enough to warrant initiation of treatment, as a clinician must confirm with appropriate studies that venous thrombosis or pulmonary embolism is present. If studies of the lower extremities are negative, then a high probability ventilation perfusion scan (V/Q) or a positive pulmonary angiogram must be obtained before therapy should be considered. However, a high probability V/Q scan occurs in only 40% of patients with angiographically proven pulmonary embolism. Thus, if suspicion is high enough, do not accept a V/Q scan of 'indeterminate probability' as negative for pulmonary embolism. Even more important, though, is prevention of the disease with appropriate measures and that every individual who is at medium to high risk for development of deep venous thrombosis must have some kind of prophylaxis.