Twenty-five cases of diffuse pigmented villonodular synovitis of the knee in 23 patients were reviewed to identify characteristic clinical presentations and histologic findings. All cases met strict histologic criteria for diagnosis. Clinical presentation consisted of insidious and progressive symptoms of significant joint swelling with discomfort, but not isolated pain. On physical examination, a suprapatellar pouch that was markedly distended by either effusion, synovial masses, or both, and a slight flexion posture were the most consistent findings. Aspiration to detect a bloody effusion was unreliable as a diagnostic tool. Pigmentation was absent in 16% of the gross specimens. Histologically, iron deposition was present in all cases and to a moderate or marked degree in most. Fibrosis, hyalinization, and chronic inflammatory changes were present in most, but to a lesser degree. No patient had gross, histologic, or treatment follow-up findings consistent with neoplasia. Attempts to predict clinical outcome based on histologic findings were unrewarding.