Failure of a medial unicompartmental arthroplasty may be related to wear of the cartilage in the opposite compartment or to wear in the polyethylene tibial implant. Limb alignment influences both of these factors in the long term. Fifty-eight knees with medial unicompartmental arthroplasties in patients alive at least 10 years postoperatively were evaluated for radiographic changes at their most recent followup. The average duration of clinical and radiographic followups for these patients was 15 years (range, 10–20 years). Alignment was measured preoperatively and postoperatively as the hip-knee-ankle angle on radiographs of the entire limb. An overcorrection in valgus of the preoperative deformity (hip-knee-ankle angle > 180°) was associated with an increased risk of degenerative changes in the opposite compartment. Severe undercorrection in varus of the deformity (hip-knee-ankle angle < 170°) was associated with increased wear in the tibial component and recurrence of the deformity which was indicative of polyethylene wear. For medial implants that were implanted in moderate varus (hip-knee-ankle angle of 171° to 179°) the rate of wear of the polyethylene was less than in knees with severe undercorrection and the risk of degenerative changes in the opposite compartment was low.