In a double-blind prospective randomised study we examined the effects of Diclofenac on heterotopic ossification after hip arthroplasty. Either the drug, or a placebo, was given by mouth to 158 patients in doses of 3 x 50 mg for 6 weeks. Diclofenac resulted in highly significant improvement (p less than 0.0001 versus controls) without severe side-effects. Heterotopic ossification decreased from 55% in the placebo to 15%. Significant degrees of heterotopic ossification did not occur with the drug. Movement of the hip was considerably increased after operation. We recommend Diclofenac prophylaxis against heterotopic ossification after hip operations in view of the serious clinical effects of this complication and its quoted incidence of 15% to 80%.