The incidence of deep vein thrombosis (DVT) in 62 consecutive patients under dextran thromboprophylaxis after total hip replacement, was examined by venography of the legs on the second postoperative day. The venography was repeated on day 9 after the operation in 43 patients to study the development of thrombi. Twenty-one patients were found to have venographically deep vein thrombosis on the second postoperative day, but had no clinical signs of DVT. All thrombi were located distally in the leg and equally distributed between the operated and the non-operated leg. Distally located asymptomatic DVT were not given specific treatment. Thrombolysis between postoperative day 2 and day 9 seemed to dominate over thrombus formation and propagation. Even large thrombi in the legs may dissolve during the first postoperative week. Thus, the timing is essential for the screening of DVT.