The objective of this study was to determine the in vivo kinematics during weight-bearing and non-weight-bearing activities for subjects with a unicompartmental knee arthroplasty (UKA) designed for high flexion and implanted with minimally invasive techniques. A total of 30 UKAs implanted in 18 patients were analyzed. All patients were implanted with a medial Zimmer Unicompartmental High Flex Knee System (Zimmer Inc, Warsaw, Ind). Under fluoroscopic surveillance, each patient performed weight-bearing deep knee bend, normal gait, and passive flexion. The kinematics of the medial UKA was determined using a 3-dimensional model registration technique. On average, the implant experienced posterior femoral rollback and normal axial rotation during flexion. However, the kinematic patterns for each patient were not consistent, and the variability was high during flexion and stance phase of gait.