A 73-year-old male was admitted to the renal division of our hospital because of hypertension, proteinuria and bilateral pretibial edema. Eight years previously, he was diagnosed as being afflicted with interstitial pneumonia on the basis of a chest X-ray examination. Laboratory tests conducted during the current admission showed normocytic normochromic anemia, renal dysfunction and mild proteinuria. Total IgG was normal, but a high proportion of IgG2 was observed. M-protein in the serum was positive for both IgG lambda and Bence Jones protein (lambda type). A bone marrow biopsy showed the proportion of plasma cells to be 10.6%, but atypical cells were not found. We diagnosed the patient's condition as plasma cell dyscrasia. Light microscopy examination of a renal biopsy specimen showed moderate mesangial proliferation with a deposition of PAS-positive and Congo red negative materials in the mesangial area: nodular gomerulonephritis was seen in some glomeruli. Immunofluorescence revealed IgG and lambda light chains, strong linear staining along the glomerular basement membrane and tubular basement membrane and positivity in the mesangial area. Results of staining for IgA, IgM, fibrinogen and C3 were weakly positive in the mesangium area, while those for C4, Clq and free kappa were negative. Positive staining of IgG2 was seen by immunoperoxidase study, but the tissue was negative for IgG1, IgG3, IgG4. Electron microscopy demonstrated a dense granular deposition in the mesangial, subendothelial and peritubular area and a microfibrillar structure in the mesangial area. The diameter of the microfibrillar structure was 14 nm on the average.(ABSTRACT TRUNCATED AT 250 WORDS)