A new method for the diagnosis of pulmonary embolism and infarction is provided by serial measurements of the serum LDH (lactic dehydrogenase) activity, SGOT (serum glutamic oxalacetic transaminase) activity, and serum bilirubin concentration. The serum LDH activity was consistently elevated and the SGOT activity was consistently normal in a series of 17 patients with pulmonary embolism or infarction. In 12 of these 17 patients the serum bilirubin was significantly increased. Using this triad permits the recognition of pulmonary embolism and infarction and makes it possible to differentiate them from acute myocardial infarction and pneumonia, the 2 diseases with which they are most often confused.