Twenty-five cases of diffuse pigmented villonodular synovitis of the knee in 23 patients were reviewed to determine the results of surgical treatment. All the cases met strict histologic criteria for diagnosis. Long-term clinical follow-up data (average, 58 months) were available for all patients. One case for a patient who was treated by marginal excision recurred within one year. All other cases (initial and recurrent) were treated by total synovectomy, preserving the functional integrity of the knee. Proximal extensor realignments were performed in patients in which chronic distention had caused a redundancy of retinacular tissues. Adhesions, an early complication in eight patients, responded well to closed manipulation and did not adversely affect long-term functional outcome. The outcome was excellent in seven and good in 16 the patients. Two of the patients have had recurrences but have not had another operation. Using this technique, the recurrence rate (8%) and morbidity are significantly lower than those reported previously.