Three patients with B chronic lymphocytic leukemia and the nephrotic syndrome had atypical membranous glomerulonephritis or lobular membranoproliferative glomerulonephritis with subepithelial and subendothelial deposits containing IgG1 kappa, IgG1 lambda or IgM and IgG by immunofluorescence, respectively. A monoclonal cryoglobulin was intermittently found in the serum in one case. In two patients, kidney deposits were made of organized microtubular structures. Intracytoplasmic immunoglobulin inclusions in the two patients' lymphocytes showed a very similar electron microscopic pattern. The immunologic phenotype of leukemic lymphocytes was analogous in the three cases (with expression of CD2) and suggestive of a late maturation step. A complete remission of the nephrotic syndrome was observed after therapy with chlorambucil (and prednisone). These observations suggest a direct role of monoclonal immunoglobulins in kidney disease.