A double-blind, placebo-controlled study was done on the influence of diclofenac, a nonsteroidal anti-inflammatory drug, on heterotopic bone formation after total hip arthroplasty. Totally, 100 operations were involved, and a follow-up was performed after 1 year. There were no cases of substantial bone formation in the treated group versus two thirds of the cases in the placebo group, and some of these had pain. Because side effects of treatment with diclofenac are few and mild, we advocate prophylactic use of this drug.