The varus-valgus stability of 20 knees with unicompartmental osteoarthritis was studied in vivo at the time of total knee replacement. Intact osteoarthritic knees had an average of 11.0 degrees of varus-valgus motion. Removal of osteophytes from the osteoarthritic compartment significantly increased the motion to 13.1 degrees (P less than 0.05), while subsequent removal of osteophytes from the nonosteoarthritic compartment further increased motion to 14.7 degrees (P less than 0.025). In primarily unicompartmental osteoarthritis, marginal osteophytes appear to stabilize osteoarthritic knees, but can cause fixed deformity.