In 1983, reliable data on the incidence of pulmonary embolism in the adult population and in groups identified at risk were not available because the sensitivity and specificity of the diagnostic procedures had not been determined. Estimates suggested there were about half a million episodes of pulmonary embolism in hospitalized patients each year in the United States. Deaths attributable to pulmonary embolism would be expected in about one third of these patients if left untreated.