Renal biopsy specimens from 20 patients, 14 women and 6 men, with rheumatoid arthritis (RA) of at least 6 months' duration were studied by direct immunofluorescence microscopy for the presence of immune deposits. Pathological changes were most prominent in patients with longstanding or malignant disease and in patients previously on gold salt therapy. Staining for IgG and C3 was negative in 4 patients with an arthritis of 2 year's duration or less. Fifteen patients had glomerular deposits containing IgG or C3, or both. In 6 of these patients staining was also positive for IgM and in 3 for IgA. In 8 patients C3 was distributed in irregular deposits along the tubular basement membrane. These results suggest that in patients with RA, immunological processes have an untoward effect on the kidneys. The long-term effects of these processes might contribute, moreover, to the development of late renal complications. There is no evidence, however, that the renal immune deposits documented so frequently in this study derive from the rheumatoid disease itself.