Fifty-six consecutive knees were operated on for single compartment disease using the Marmor modular knee with a minimum follow-up of four years. The information obtained indicates that single compartment replacement is valuable in restoring function and relieving pain in a high percentage of patients. There was no evidence of late degeneration of the successful results at the time of writing. No difference in the results obtained with medial or lateral compartment replacement has been noted. Degeneration of the compartment not operated on has not been a problem. Unicompartmental replacement is an excellent procedure in the elderly patient who has osteoarthritis of the knee with varus or valgus deformity.