Twenty-five patients who had had a diagnosis of pigmented villonodular synovitis of the knee were followed for an average of four and one-half years (range, two to ten years) after arthroscopic treatment. Five patients had had localized lesions and had been managed with local resection; all five had improvement, with no apparent recurrence. The remaining twenty patients had had diffuse disease. Of these twenty, eleven had had a complete arthroscopic synovectomy. All eleven had definite improvement in pain and function, and almost all had a decrease in synovitis and an increase in the range of motion of the knee; the disease recurred in only one. The other nine patients had had a partial arthroscopic synovectomy. Although most had some improvement in function and range of motion and a decrease in pain and synovitis, the disease recurred in five of the nine. Thus, in the patients who had had diffuse pigmented villonodular synovitis, the rate of recurrence was lower in those who had had a complete arthroscopic synovectomy than in those who had had a partial arthroscopic synovectomy (p = 0.01).