1 Two multicentre, parallel group, randomised, double-blind, double-dummy comparison studies were conducted between isoxicam in the usual dose of 200 mg once daily and naproxen 500 mg twice daily. 2 The drugs were administered for 4 weeks to 230 patients suffering from osteoarthritis of the hip and/or knee in the first trial and to 249 patients suffering from rheumatoid arthritis in the second. 3 The studies compared treatments for both safety and overall effectiveness in the relief of pain. 4 In the osteoarthritis trial, overall pain was reduced by both drugs after 2 weeks of therapy but only isoxicam produced further improvement after 4 weeks. 5 Isoxicam produced reductions comparable to those produced by naproxen in pain on standing from the sitting position, pain on walking, and pain on movement of the affected joint, after 2 and 4 weeks. 6 After 4 weeks, isoxicam given once daily in the morning was significantly more effective than naproxen given in the morning and the evening in relieving not only total pain as assessed by a visual analogue scale but, as importantly, night pain. 7 Compared to naproxen therapy, isoxicam therapy was associated with significantly more patients whose disease state was improved at 2 weeks, as assessed by physicians. 8 In the rheumatoid arthritis trial, isoxicam was equally as effective as naproxen in reducing joint tenderness, joint swelling, and pain; at 4 weeks there was a trend in favour of isoxicam in reduction of joint swelling and pain. 9 Isoxicam reduced morning stiffness significantly more than naproxen after 4 weeks; this trend was apparent at 2 weeks.(ABSTRACT TRUNCATED AT 250 WORDS)