Deep vein thrombosis (DVT) is a common complication in patients undergoing elective hip surgery. Because of the associated risk of pulmonary embolism, prophylaxis with standard (unfractionated) heparin is becoming increasingly important. Recent clinical trials have shown a low molecular weight form of heparin, enoxaparin, to be more effective than standard heparin in preventing DVT, but the new drug is also more expensive. Data on clinical effectiveness and cost were combined in an economic evaluation of the two regimens. It was found that prophylaxis with enoxaparin would be expected to lead to a net saving of £20 per patient. The economic results are sensitive to the costs of enoxaparin, the costs of drug administration and the probability of false clinical diagnosis of DVT or pulmonary embolism.