Signs of glomerulopathy, especially a nephrotic syndrome can occur in cancer patients, but the exact frequency of glomerular lesions is not well known in these patients. To define this frequency in a given type of malignancy we have studied the nephrectomy kidneys in 40 patients with renal cell carcinoma. Proteinuria, which was present in 7 cases, ranged from 0.15 to 1.5 g per 24 h. Reduction of the creatinine clearance greater than 50% was observed in 5 patients. Circulating immune complexes were detected in 11 of the 15 patients studied. Carcinoembryonic antigens were noted in 2 of 9 patients investigated. Research of alpha 1 foetoprotein carried out in 12 patients was always negative. HBs antigen or Hbs antibodies were detected in 6 of 29 patients studied. Light microscopic examination of the normal uninvolved kidney tissue showed obvious glomerular lesions (mesangial hypertrophy with or without deposits, with or without cell proliferation) in 7 patients (17.5%). Amyloid deposits were never observed. Immunofluorescence study revealed mesangial deposits in 35% of patients versus 5.4% of control subjects (P less than 0.0001). These deposits included C3 and/or IgM in 13 cases, IgA and C3 in one case. No fixation was observed, neither on tubules of normal tissue nor on carcinoma lesions. This report demonstrates that glomerular deposits are usually found in approximately one third of patients with renal cell carcinoma and that these deposits are located in the mesangial areas and not in the subepithelial space as it is often observed when glomerulonephritis is expressed by clinical symptoms.