Thirty-five female patients, mean age 63 years, suffering from rheumatoid arthritis participated the study. Twenty patients had been on long-term low-dose corticosteroid treatment. Fifteen patients had never received corticosteroids. A control group of 15 age- and sex-matched healthy subjects was also studied. Examination of muscle biopsies from the (right) vastus lateralis and measurements of isokinetic and isometric knee-extension muscle strength were performed in all subjects. Rheumatoid arthritis patients treated with corticosteroid showed a low percentage of type I fibres, mean 35.7 (range 17-66) % compared with patients who did not receive corticosteroid (P less than 0.005). The latter group did not differ from the controls. The muscle fibre areas also were affected in the corticosteroid treated rheumatoid patients. Type I and type II mean fibre areas were reduced by 32% and 50%, respectively, when compared with non-prednisone treated patients. The latter group did not differ from the controls in this respect. A correlation was found between the isokinetic muscle strength of the knee extensors and the mean areas of type I and type II in patients treated with prednisone (r = 0.48, P less than 0.05 and r = 0.58, P less than 0.02 respectively). No such correlation was found when using isometric measurements of the knee extensors. A positive correlation was found in both groups of rheumatoid arthritis patients between the areas of the type I and type II fibres (r = 0.66 - 0.68, P less than 0.05 - 0.02).(ABSTRACT TRUNCATED AT 250 WORDS)