A 44-year-old man demonstrated proteinuria, microhematuria and renal dysfunction during the course of hyperthyroidism. Neither cryoglobulinemia nor paraproteinemia including light chains was found. No systemic signs suggestive of amyloidosis appeared. Histological findings showed a diffuse mesangial matrix increase with slight mesangial proliferation and diffuse granular depositions of IgG, IgA, C3, C4, Clq, k and λ light chains. Ultrastructurally, micro fibrils of about 20 nm in width were seen to be deposited diffusely in mesangial areas and in glomerular basement membranes. Congo red and thioflavin T staining were negative. These findings support the possible existence of a specific glomerular disease different from amyloidosis.