Thromboembolism is the most common serious complication following total hip arthroplasty and most common cause of death after total hip arthroplasty. A prospective randomized study in 170 cases of elective cementless total hip arthroplasty was carried out to examine the incidence of deep vein thrombosis and pulmonary embolism after cementless total hip arthroplasty from Aug. 1993 to May 1995. Laboratory study, clinical symptoms and signs, chest roentgenograph and precipitating factors were analysed. Venography and lung perfusion scan using radionuclide scan were used for this study. The weight, height, sex, habitus of alcohol and smoking, hypertension, diabetes mellitus, previous operation history of ipsilateral lower extremity, etiology of hip joint disease, and transfusion of blood were not precipitating factors, but the age over 40 and previous history of pulmonary embolism had a significant effect on the incidence of deep vein thrombosis. There was no significant relationship between the incidence of deep vein thrombosis and the laboratory assay, clinical symptoms and signs. Deep vein thrombosis was detected in 29 cases(17.0%), pulmonary embolism in 22 cases(12.9%), and fatal pulmonary embolism in 1 case(0.6%). The most common location of deep vein thrombosis was the popliteal area.