Severe nephrotic syndrome developed in an 83-year-old Japanese man with Waldenström’s macroglobulinemia. Treatment with corticoid remarkably improved the proteinuria. Autopsy disclosed no deposit of amyloid in the kidneys but a slight infiltration of atypical lymphocytes. A considerable number of glomeruli showed mesangial cell proliferation and global or segmental thickening of the basement membrane with occasional double tracks. Immunofluorescent studies revealed deposits of IgM but not IgG, IgA and C<sub>3</sub> along the basement membrane in most glomeruli. Electron microscopy disclosed the splitting glomerular basement membrane and scattered electron-dense deposits.