Maximum isometric muscle strength on extension and flexion of the knee joints and endurance on extension has been studied in patients with rheumatoid arthritis (RA) and osteoarthrosis (OA), with such severe degeneration of the knees that arthroplasty was indicated. This material was compared with a group of healthy volunteers. The reduction in muscle strength was most pronounced in the RA group, which had 30-45% of the strength of the healthy volunteers. In the OA group the strength, similarly compared, was 55-70%. The difference is explained by the effect of the rheumatic disease on the muscles. The results indicate that in therapeutic measures due to trauma or in joint surgery, methods that contribute to a further decrease in muscle strength, e.g. immobilization with casts, bandages, or traction treatment, should be avoided.