A double-blind trial compared diclofenac with placebo in 44 outpatients. On from each group dropped out with dyspepsia, and one (placebo group) with ineffective treatment. Twenty completers received diclofenac. Dosage was one tablet (25 mg diclofenac) three times daily during the first week. In the second (final) week, most patients had four or six tablets. Diclofenac had significantly greater effect on pain, grip, morning stiffness, joint tenderness and swelling, and in comparison to previous treatments, even though the placebo group required significantly more rescue anaglesic. A few patients in each group had slight dyspepsia. One in the active and six in the placebo group complained of minor central nervous system symptoms. There were no serious side-effects. Haematological, biochemical and urinary analyses showed no clinically important changes. It is concluded that, in the short term, diclofenac (Voltarol) is effective in relieving the symptoms of inflammatory polyarthritis. It is well tolerated as placebo medication, and had no detrimental haematological or biochemical effects.