A randomized blind clinical trial was carried out to assess the effect of dihydroergotamine (DHE) alone and in combination with heparin on the incidence of thromboembolic complications of total hip replacement. It was found that a combination of DHE 0.5 mg and heparin 5000 units subcutaneously, given 8-hourly in separate injection sites, reduced the incidence of thromboembolic complications from 63 per cent in the untreated controls to 7.4 per cent in the treated group. This difference was highly significant. DHE alone produced a reduction in the incidence of thromboembolism but this reduction was not statistically significant. Blood loss was calculated using pre- and postoperative haemoglobin levels, taking into account the amount of blood transfused. It was found that the blood loss in the DHE/heparin-treated patients was identical to that in the untreated controls. The patients who received DHE alone had a significantly lower mean blood loss than the other two groups. It is concluded that this reduction in blood loss by DHE may be due to its venoconstrictor properties.