The purposes of this study were to describe a method for measuring tibiofemoral subluxation in UKA, and to report the mean amount of tibiofemoral subluxation seen both preoperatively and postoperatively in a cohort of patients undergoing UKA. Two hundred thirty-five patients who received a medial UKA, and 39 patients who received a lateral UKA, were reviewed. In the medial UKA cohort, the mechanical alignment was corrected from 7.7°±5.9° preoperatively, to 2.9°±2.5° postoperatively, while the tibiofemoral subluxation was corrected from 4.5 mm±3.0 mm preoperatively, to 2.3 mm±2.7 mm postoperatively. In the lateral UKA cohort, the mechanical alignment was corrected from −5.5°±3.8° to −1.6°±3.4°, while the tibiofemoral subluxation was corrected from 4.3 mm±2.7 mm to 2.8 mm±2.5 mm. This study presents a novel method for measurement of tibiofemoral subluxation, the mean amount of tibiofemoral subluxation present preoperatively, and the amount of correction that can be expected during both medial and lateral unicondylar knee arthroplasty.