The incidence and importance of bland pulmonary embolism as a cause of morbidity and mortality have been emphasized. The pathologic physiology, the clinical picture, the laboratory findings, the diagnosis, and the treatment have been discussed. Venous thrombosis in the lower extremities is the most common immediate cause of pulmonary embolism, but clinical evidence of its presence is frequently lacking. It is difficult to prevent venous thrombosis in spite of the employment of numerous prophylactic measures. Although the diagnosis of pulmonary embolism is easy in some cases, it may be impossible in others. Statistical proof of the effectiveness of treatment is difficult to obtain, but clinical experience appears to indicate that the aforementioned therapeutic procedures are worthwhile. It is evident that many important problems relating to thromboembolism remain unsolved.