We studied the incidence of deep vein thrombosis in the 110 Korean patients who had Porous Coated Anatomic (P.C.A.) cementless total hip replacement. The diagnosis of deep vein thrombosis was made by roentgenographic venography. The perfusion lung-scanning was done to make a diagnosis of pulmonary embolism. Our findings showed that there was an unusually low incidence (7 per cent) of deep vein thrombosis in our unprotected patient group. They also revealed that some factors that are believed to be relevant to thrombosis were conspicuously rare in this series. In view of this fact, we re-evaluated the so-called risk factors and came to a conclusion that some of them-such as advanced age, venous valve number; coagulation assay data, orthopedic diagnosis, preoperative limitation of mobility, hypertension and blood group-that have been claimed to be relevant in fact seem to be irrelevant to deep vein thrombosis.