A six month single-blind, randomized parallel group study comparing the effects of fenclofenac and diclofenac in 63 patients with rheumatoid arthritis is described. Both treatments produced improvements in clinical measurements, with a significant between-treatment effect in favour of fenclofenac for overall pain, night pain and duration of morning stiffness. Both treatments produced a decrease in IgM and the fenclofenac group produced decreases in plasma viscosity and ESR, the latter showing a significant between-treatment effect in favour of fenclofenac. No clinically significant changes in routine haematology and biochemistry were noted. Unwanted effects leading to withdrawal of therapy were reported by five patients in the fenclofenac group and three patients in the diclofenac group. Two patients in the fenclofenac group and one in the diclofenac group were withdrawn for reasons unrelated to therapy. In the diclofenac group two and four patients were withdrawn for clinical deterioration and inadequate effect respectively.