Two well-matched groups of patients with unicompartmental knee arthroplasties were compared. The first 51 knees were treated with a fixed-bearing knee implant and the second 50 knees were treated with a mobile meniscal-bearing implant. Followup was 7.7 years for the patients with fixed-bearing implants and 6.8 years for patients with mobile-bearing implants. Both groups functioned well clinically. Radiographic analysis with 3-foot standing views taken preoperatively showed both groups had an average varus alignment of -2 degrees. Postoperatively patients with fixed-bearing implants had an average +2.6 degrees alignment and the patients with mobile-bearing implants had +5.5 degrees alignment, which was significantly different. Survivorship analysis based on component loosening and revision showed a 99% survival for the meniscal-bearing implant and 93% survival for the fixed-bearing implant at 11 years. However, the fixed-bearing knee implants failed significantly more often because of tibial component failure, in six of eight knees, at an average of 6.3 years. The mobile-bearing implants showed a trend to fail because of arthritic degeneration in the lateral compartment, at an average of 10 years, although not statistically significant. The mobile-bearing implants had no tibial component failures. These differences may be attributable to implant design or surgical technique.