In the knees of twenty-five patients with rheumatoid arthritis who had had synovectomy an average of thirty-one months previously (range one to 120), arthroscopy, aspiration, and biopsy were done. The findings were that the regenerated synovium became histologically indistinguishable from that found in patients not operated on—grossly and microscopically active areas alternated with inactive ones, lysosomal glycosidases were elevated and rose with time, and the synovium of eighteen of twenty-one of the patients two years after synovectomy met the criteria for rheumatoid disease.