The incidence of clinically significant pulmonary embolism following total hip arthroplasty is reported in patients with venographically documented deep venous thrombosis. One hundred seventy-four patients who enrolled in a prospective evaluation of deep venous thrombosis prophylaxis underwent contrast venography 7-10 days after operation. Ten patients with proximal and 12 patients with isolated deep calf thrombosis were identified on routine venogram reading and received appropriate anticoagulant therapy, including a 6-12-week course of warfarin. There were no clinically evident embolic events and no bleeding complications in this group. A secondary blinded review of venograms several weeks later identified isolated deep calf thrombi in 13 other patients, none of whom had received warfarin after discharge. Four of the 13 patients (P < .02) subsequently presented with pulmonary embolism at a mean of 33.5 days after operation. Asymptomatic deep calf venous thrombosis following total hip arthroplasty is associated with a significant risk of developing clinically evident pulmonary embolism within the first 8 weeks after operation. Once identified, these thrombi were effectively managed with outpatient warfarin anticoagulation, and both embolic and bleeding complications were avoided. This study documents the need for more prolonged routine prophylaxis against venous thrombosis than has heretofore been the rule, extending beyond the time of discharge from the hospital. Alternatively, the authors' experience supports the use of routine screening venography following total hip arthroplasty to allow detection and selective anticoagulant treatment of deep venous thrombosis in both the thigh and calf.