The operative treatment of medial compartment knee arthritis is controversial. The purpose of the present study was to report the experience of a single surgeon with the UniSpacer arthroplasty for the treatment of isolated medial compartment arthritis of the knee.From April 2002 through November 2002, thirty-seven UniSpacer arthroplasties were performed in thirty-four patients for the treatment of arthritis that primarily involved the medial compartment of the knee. The Ahlbäck radiographic evaluation scale was used to grade the severity of arthritis; the mean preoperative score was 2.6 points for the medial compartment and 0.5 point for both the lateral and patellofemoral compartments. The study group included eighteen women (nineteen knees) and sixteen men (eighteen knees) who had a mean age of fifty-five years (range, forty-two to seventy-five years) at the time of surgery. Twelve patients had had a previous arthroscopic meniscectomy. The mean preoperative Knee Society function score was 60 points (range, 40 to 80 points), and the mean preoperative Knee Society objective score was 62 points (range, 40 to 76 points).After a mean duration of follow-up of twenty-six months (range, twenty-four to twenty-nine months), there were no excellent, ten good, fifteen fair, and twelve poor results. The mean postoperative total function score was 69 points (range, 40 to 82 points), and the mean Knee Society objective score was 72 points (range, 45 to 88 points). Six of the twelve poor results were in knees that had dislocation of the UniSpacer. All twelve knees were revised to a total knee arthroplasty.On the basis of this experience, we do not recommend UniSpacer arthroplasty for the treatment of degenerative arthritis of the medial compartment of the knee.