1. The literature on pigmented villonodular synovitis has been reviewed and a series of eighty additional cases is reported.
2. The condition usually presents either as a nodule in a finger or knee, or as a diffuse lesion in a knee. The lesions, although benign, sometimes erode or invade the tissue of adjacent bones.
3. Distinction from malignant synovioma can be made on the basis of the macroscopic appearance of the lesion at operation (relationship to joints or tendon sheaths: villonodular appearance: pigmentation), and by histological examination.
4. Treatment of the nodular form by excision is satisfactory but extensive synovectomy for diffuse lesions of the knee gives poor results.
5. The etiology of pigmented villonodular synovitis is unknown, but it appears to be a self-limiting process, possibly inflammatory in nature.