We reviewed the results of all eighteen total knee arthroplasties that had been performed at the Mayo Clinic between 1974 and 1992 for the treatment of pigmented villonodular synovitis. The diagnosis had been confirmed histologically in all patients. Fourteen patients had diffuse disease, which was active in eleven and inactive in three, and four had focal disease. In addition to the arthroplasty, the eleven patients who had active diffuse disease were managed with a complete synovectomy and the four patients who had focal disease had a partial synovectomy. The three remaining patients had a history of diffuse pigmented villonodular synovitis but had no evidence of disease at the time of the arthroplasty, so a synovectomy was not performed. At an average of 9.9 years (range, 3.6 to 20.1 years) after the arthroplasty, fourteen of the eighteen total knee prostheses were well fixed and functioning satisfactorily. The average knee score was 89 points (range, 80 to 99 points) and the average functional score was 77 points (range, 20 to 100 points) for these fourteen patients. The four failures were in patients who had active diffuse disease at the time of the arthroplasty. Three of the failures were due to aseptic loosening, and one was due to recurrence of the disease. Sixteen patients had no evidence of recurrence when they were last seen, at an average of 10.3 years (range, 3.6 to 20.1 years) after the arthroplasty. The two patients who had a recurrence were among the eleven who had active diffuse disease. In one of these patients, the recurrence was diagnosed at the time of a revision arthroplasty performed because of aseptic loosening. The other patient had two recurrences, necessitating an above-the-knee amputation.