We reviewed the outcome of 30 consecutive primary unicompartmental knee arthroplasties (UKAs) performed by a single surgeon over a 26-month period. All operations were performed to treat osteoarthritis of the medial compartment of the knee. Fifteen Allegretto (Sulzer, Winterthur, Switzerland) UKAs were implanted without computer navigation whereas 15 EIUS (Stryker-Howmedica, Allendale, NJ) UKAs were implanted using navigation. The patients were assessed clinically using the Oxford knee score and radiologically using long-leg weight-bearing films and non-weight-bearing computed tomography leg alignment films. No patients operated on were lost to follow-up. Unicompartmental knee arthroplasty performed with computer-assisted surgical navigation resulted in a more accurate and reproducible limb alignment than UKA performed without surgical navigation.