Five years ago, in October 1963, the first lung scans carried out in man were presented at the Symposium on Dynamic Clinical Studies with Radioisotopes in Oak Ridge, Tenn. Since then, the procedure has become widely adopted as a diagnostic and research tool. While still short of being infallible, scanning of the lungs has been most helpful in the differential diagnosis of pulmonary embolism. At the same time, it has expanded our knowledge of the natural history of the disease and is aiding in the development of promising new therapies. One of the things that scanning has taught us about pulmonary embolism is that it is much more common and—on the average—rather less serious than was once thought. Some recent postmortem studies have shown evidence of emboli in as many as half the adult patients dying from all causes. It seems probable that many emboli are never detected because they are asymptomatic and, for the most part, resolve spontaneously with no known ill effects. As yet there is no single procedure that is sensitive, specific, safe, and inexpensive enough to serve as a screening test. The physician must therefore rely heavily upon the patient's history and the presenting symptoms. Pulmonary emboli usually originate as thrombi in the pelvic region or, less commonly, in the lower extremities. They occur most frequently postoperatively, postpartum, and, indeed, in any situation in which the patient has been more or less immobilized and has become predisposed to clot formation. With a suspicious history and symptomatology, the next step is generally a chest film. This can be useful, but most often not in the way one might suspect. In only about 20 per cent of even severe embolism cases are x-ray findings initially positive. Usually these are indicative of pulmonary infarction, which is most likely to occur in connection with congestive failure or systemic hypotension. In other patients negative roentgen findings together with a positive scan will actually strengthen the diagnosis of an embolism, since they can help to ride out alternatives such as early pneumonia. In discussing the interpretation of lung scans, it is important to define precisely the information they yield. They do not detect emboli as such; what they do show is the distribution of pulmonary arterial blood flow in the lungs. Whatever abnormalities exist in that distribution must be interpreted in the light of their specific form and location, the changes they may or may not undergo with time, and the patient's other signs and symptoms. It is also worth noting that scanning cannot pick up lesions less than 2 to 3 cm in diameter. A small lesion of this sort may produce a peripheral infarct, with pleural pain and perhaps hemoptysis. Here the scan may be normal, so that diagnosis must be made on other grounds.