In a double-blind, crossover study conducted in ten hospitalised patients suffering from active rheumatoid arthritis, the anti-inflammatory effect of diclofenac sodium (Voltaren) in a daily dosage of 125 mg was compared with that of placebo under strictly standardised conditions. Changes in reversible articular swelling were determined by measuring the circumference of involved wrist, knee, metacarpophalangeal, interphalangeal, and metatarosphalangeal joints. A consistent decrease in joint swelling occurred in response to treatment with diclofenac sodium, as compared with a consistent increase following placebo medication. This difference was significant at the 5% level for all except the interphalangeal joints. Similarly, in response to diclofenac sodium a greater improvement was noted in duration of morning stiffness and grip strength and a greater reduction in the total number of painful and swollen joints. The status of the rheumatoid condition improved in nine patients following treatment with diclofenac sodium, but in none following placebo medication. Two patients reported unwanted effects, consisting of a sensation of fullness during treatment with diclofenac sodium in one case and of heartburn during both treatment periods in the other. No laboratory abnormalities were noted.