A patient with Walderström macroglobulinemia associated with nephrotic syndrome is described. Serum cryoglobulin and rheumatoid factor were absent. Intramembranous electron-dense deposits were demonstrated in kidney biopsy material by electron microscopy. Deposits of immunoglobulin G (IgG), M (IgM) and the third component of complement (C3) were identified in kidney biopsy tissue by immunofluorescent staining methods. The serum immunoglobulins were characterized by chromatographic and immunochemical methods and showed a monocional IgM-K, IgG-K and gamma-chain piece of undefined structure. Free K- and gamma-chains were found in the urine. The IgM was not complexed to the IgG or vice versa, but the IgG was in an affregated form. Although it is not known which immunoglobulin initiated the tissue injury, IgG, IgM and complement deposits probably contributed to the renal dysfunction. The nephrotic syndrome diminished after treatemnt with chlorambucin and corticosteroids.