The effect of tenoxicam 10 mg and 20 mg, administered daily for 6 weeks to prevent heterotopic bone formation after total hip arthroplasty, was evaluated in a randomized, double-blind, placebo-controlled trial involving 90 patients. After 3 months, patients who had received the active drug, including those who had received only half the recommended anti-inflammatory dosage, had significantly less heterotopic bone formation. After 6 months the difference between treatment groups and placebo became smaller but remained significant. Adverse reactions occurred in only 3 patients, reflecting no differences between the groups. The study results, including radiographic, clinical and biochemical evaluations, demonstrate that treatment with tenoxicam 20 mg daily and even with tenoxicam 10 mg daily for 6 weeks, starting immediately after total hip arthroplasty, is effective in preventing ectopic bone formation.