The results of arthroplasty of the knee joint in 24 patients with rheumatoid arthritis using the Macintosh prosthesis have been reviewed. Twenty-eight operations were performed in patients with severe disabling pain in the knee, well-marked radiological changes, and usually with flexion and valgus deformities. By our criteria only 42% achieved a good result after an average follow-up of three years. Possible reasons for this poor success rate are discussed. In the absence of a satisfactory total knee replacement so far, this arthroplasty may be considered as an alternative to arthrodesis in certain patients with rheumatoid arthritis. Our data indicate that those with multiple joint involvement, severe disabling pain in the knees with flexion and valgus deformities not greater than 30 degrees have a 50% chance of a good result.