The efficacy of naproxen in treating rheumatoid arthritis patients was evaluated in a double-blind clinical trial using aspirin as the control drug. The study was conducted at seven centers and involved 80 patients. After an unequivocal increase in disease activity during a drug-free period, patients were randomly assigned to either drug and continued in the trial for 16 weeks. Some patients took low maintenance doses of corticosteroids, or gold salts, or both throughout the trial. Both test drugs significantly decreased disease activity as measured by a number of ways. By objective measurements, naproxen was as effective as aspirin, although patients in the naproxen-treated group entered the trial with more severe disease. By some subjective evaluations, naproxen was considered more effective than aspirin. Although patients taking naproxen had less frequent gastrointestinal side effects and fewer symptoms VIIIth nerve toxicity, the differences were not statistically significant. We conclude that naproxen is a useful addition to the physician's armamentarium for the treatment of rheumatoid arthritis.