The most common previously reported modes of failure of unicompartmental knee arthroplasty (UKA) in the first and second decades are polyethylene wear, progression of arthritis, and component loosening. The purpose of this study is to describe an early mechanism of failure of the medial UKA. Thirty-two consecutive revisions from UKA to total knee arthroplasty were retrospectively reviewed. The predominant mode of failure observed in 15 (47%) of 32 knees was medial tibial collapse. Of these, 87% were an all-polyethylene design, and 7 of 15 failed in less than 16 months and required more complex reconstruction with stems, augments, and screws and cement. Increased tibial slope was associated with posterior tibial collapse. In our series, knees that failed by medial tibial collapse had more significant bone defects and required more complex reconstructions than is currently reported in the literature.