Pregnancy has been recognized as a predisposing risk factor of deep venous thrombosis(DVT) and pulmonary thromboembolism(PTE). Recently, we investigated the incidence of PTE between 1991 and 2000 among 64 facilities of Obstetrics and Gynecology in Japan. As a result, incidences of PTE are 0.006% (14/227,318) in cases of vaginal delivery and 0.046%(25/54,277) in cases of cesarean section (CS) in Obstetrics. In Gynecology, they are 0.03% (34/106,598) in patients of benign tumors and 0.33%(54/16,206) in patients of malignant tumors. Classification of level of risks in Japanese subjects at present is thought to be as follows; cases of normal delivery and minor surgery are low risk, cases of CS and benign gynecological tumors are moderate risk, cases of CS with elderly fatty pregnant women and gynecological malignant tumors are high risk, and cases of CS or malignant tumors with thrombophilia or pre-DVT are the highest risk. Prophylaxis of DVT should be recommended early ambulation and adequate hydration in low risk cases, graduated compression stocking (GCS) or intermittent pneumatic compression (IPC) in moderate risk cases, IPC or the use of low-dose unfractionated heparin(LDUH) (and/or GCS) in high risk cases, and LDUH and IPC or LDUH and GCS in the highest risk cases.