Glomerulonephritis has been believed to be a rare complication in rheumatoid arthritis (RA). However, recent studies have revealed a focal segmental increase in mesangial cells and matrix in RA patients with hematuria. In our series, proteinuria, hematuria or both abnormalities were recognized in 74 (22%) out of 336 RA cases. Among 119 patients examined by renal biopsy, mild mesangial proliferative glomerulonephritis (GN) was found in 25 patients, of which 22 demonstrated mesangial IgA deposits, by immunofluorescent microscopy. Membranous nephropathy was noticed in 26 cases. Three cases of membranous nephropathy had no history of gold or D-penicillamine treatment. Electron microscopy revealed diffuse thinning of the glomerular basement membrane in 12 cases. The average thickness of the glomerular basement membrane was significantly thinner in RA patients than in normal subjects. The immunological processes associated with rheumatoid factor do not seem to be related to the renal lesions in RA patients.