In a randomized, double-blind, placebo-controlled multicenter trial, the efficacy and safety of dihydroergotamine mesylate/heparin sodium as a prophylactic agent for deep-vein thrombosis were evaluated in 148 patients who were forty years old or more and who underwent total hip replacement. The incidence of venographically proved postoperative deep-vein thrombosis was 52 per cent in the placebo group and 25 per cent in the dihydroergotamine mesylate/heparin sodium group (p = 0.002). Proximal thrombi developed in only 5 per cent and extensive thrombi, in only 10 per cent of the patients who received dihydroergotamine mesylate/heparin sodium. In contrast, proximal thrombi and extensive thrombi developed in 19 and 25 per cent, respectively, of the patients in the placebo group (p less than 0.05). Adverse reactions in the two groups did not differ significantly: in the treatment group they consisted primarily of hematoma at the site of injection (9 per cent), hematoma at the wound (5 per cent), and excessive postoperative bleeding, and in the placebo group there was hematoma at the site of injection (3 per cent). It was concluded that the combination agent dihydroergotamine mesylate/heparin sodium was effective and safe prophylaxis against deep-vein thrombosis for the patients who underwent total hip replacement in this study.