Hip and knee replacement arthroplasty is one of the important causes of pulmonary embolism, of which the incidence was reported to be from 2 to 16% But, there are no reports about the incidence of pulmonary embolism after these operations in Korea. The authors' intention was to study the incidence and clinical manifestations of pulmonary embolism after hip and knee surgery. Lung perfusion scans and inhalation scans were done preoperatively and 7 days postoperatively for 139 hip or knee replacement arthrosplasty patients in Asan Medical Center. We studied 62 cases of avascular necrosis of the femoral head, 33 cases of femur neck fractures, 45 cases of degenerative arthritis of the hip and knee joints and 17 cases of other diseases. In this study, hip replacement arthroplasties were done in 114 cases, knee replacement arthrosplasties in 25 cases. On preoperative lung scan, high or intermediate probability findings of pulmonary embolism were shown in 18 cases and all of them were asymptomatic. 139 cases showed normal or low probability on preoperative lung scan, of which 5 cases were changed to high probability and 7 cases to intermediate probability on postoperative lung scan. Four of the 7 intermediate probability cases were confirmed to be pulmonary embolism on pulmonary angiography. Of these 9 cases, hip replacement arthroplasties were done in 6 cases and knee replacement arthroplasties in 3 cases. There were 5 cases of osteoarthritis of hip and knee, 3 cases of avascular necrosis of femoral head and 1 case of femoral neck fracture. We concluded that preoperative and postoperative incidences of pulmonary embolism in Korea were similar to those in western countires. Most of the pulmonary embolism patients were asymptomatic and so lung perfusion scans and inhalation scans were helpful in the early diagnosis of pulmonary embolism.