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J. Lima, Service of Internal Medicine, Hospital General “Valle Hebron’, Paseo Valle Hebron S/N, E-08035 Barcelona (Spain) Dear Sir, Renal involvement in benign monoclonal gammopathy (BMG) is rare [1]. Although the histological findings in renal biopsies are very varied, we found only 1 case of BMG and glomerulonephritis (GN) with endoand extracapillary proliferation [2]. We studied a patient with IgM BMG and GN, in whom endoand extracapillary proliferation was revealed by renal biopsy. Renal function improvement with immunosuppressive treatment is stressed. A 76-year-old man was admitted because of bilateral leg edema of 2 months duration. He had no previous medical history. Physical examination showed no particular abnormalities except for malleo-lar swelling. Blood analysis revealed an erythrocyte sedimentation rate of 118 mm/h, hemoglobin 7.44 mmol/l (12 g/l), serum creatinine 225.4 μmol/l (2.55 mg/dl), serum total protein 61.6 g/l and albumin 0.43 mmol/l (2.8 g/dl). Serum immunoelectrophoresis showed a monoclonal IgM k spike. Serum IgM levels were 30 g/l (3,000 mg/dl), while the other immunoglobulin values remained normal. Leukocyte and platelet counts, results of liver function, antinuclear antibody, rheumatic factor, cryoglobulins, hepatitis B surface antigen and complement were all normal or negative. Urinalysis revealed 80 red blood cells/high-power field and 6 g proteinuria in a 24-hour urine collection. Urine immunoelectrophoresis detected a small homogeneous IgM-κ spike; no free light chains were detected. Skeletal survey and studies for occult neoplasia were negative. Bone marrow biopsy revealed 5% mature plasma cells. Percutaneous kidney biopsy yielded 20 glomeruli. Optic microscopy demonstrated segmentary mesangial proliferation with subendothelial deposits; 40% of glomeruli had crescents (fig. 1); 8 glomeruli showed diffuse sclerosis. There was moderate interstitial fibrosis with slight round cell infiltration. Immunofluorescence showed marked segmentary granular capillary and scanty mesangial deposits of IgM (fig. 2). Anti-IgG, anti-IgA and anti-C3 were absent. Stains for amyloid were negative. Serum creatinine level rose to 397.8 μmol/l Fig. 1. Segmental hypercellularity with epitelial reaction and sinechiae. Interstitial fîbroedema with tubular basement membrane sclerosis. Periodic acid-Schiff. × 200. > ; ‘‘ ‚ f * *