Objective — the study the causes of pulmonary embolism using data of autopsies and clinical examinations. Material and methods. A retrospective analysis included data of 1136 autopsies performed at the regional clinical hospital and regional oncological dispensary of the Tula city in 1999—2005, 2042 autopsies performed at the autopsy department of the Pirogov Municipal Hospital №1 in 2011—2013. Moreover, we used data of autopsies performed at the Municipal Hospital №1 in 1964—1973 and data from all hospitals of the Moscow Healthcare Department. Clinical survey included 602 patients admitted to the Saveliev University’s Surgical Clinic with signs of pulmonary embolism in 2012—2014 (292 cases) and 2016—2017 (310 cases). Results. Autopsy. Peripheral thrombosis in the system of inferior vena cava is the main cause of pulmonary embolism (tibiopopliteal segment as a rule, 58.3%). Segmental and lobar pulmonary arteries are damaged more often (65.8%) compared with main pulmonary arteries and pulmonary trunk (p<0.05). Thrombosis of the femoral and iliac veins was detected in 196 (35.1%) cases. In 48% of these patients, main pulmonary arteries and pulmonary trunk were damaged. Thrombosis of tibiopopliteal segment followed by massive pulmonary embolism was observed less frequently (102 (46%) out of 325 autopsies with this localization of clots). Clinical study. Thrombosis of tibiopopliteal segment resulted pulmonary embolism in less than 1/3 of cases (28.1%). Thrombosis of femoral veins was more frequent cause of embolism (by 19.4%). Pulmonary embolism was caused by thrombosis of iliocaval segment in 19% of cases. Every fifth (21.1%) patient (24% of patients with massive embolism) had bilateral venous thrombosis of the lower extremities. Cause of pulmonary embolism was not found in 5.3% of clinical cases that is similar to autopsy data (4.1%). Conclusion. Thrombosis in the system of inferior vena cava is the main cause of pulmonary embolism according to autopsies and clinical examinations. Clinical diagnosis often reveals thrombosis of femoral and iliocaval segments as a cause of both massive and peripheral pulmonary embolism.