Two patients with Ig deposition disease presented with acute renal failure, moderate proteinuria, and hematuria. A plasmacytoid lymphocytic infiltrate was identified in bone marrow that produced IgG4 lambda and free lambda light chains. One patient developed an anaplastic plasmacytoma (secreting only lambda light chains) 1 yr after renal biopsy. Renal biopsy in both patients demonstrated a nodular intercapillary glomerulopathy and electron dense granular deposits, associated with a linear pattern of IgG4 heavy chain deposition in vascular, tubular, and glomerular basement membranes (VBM, TBM, and GBM). In one patient this entrapped IgG4 was unassociated with detectable kappa or lambda light chains. In the second patient, lambda light chains (1+) were detected only in the GBM, but IgG4 (4+) was identified in GBM/TBM. Neither circulating (peripheral blood and bone marrow serum) nor cellular free gamma chains were present. We propose the term "pseudo-gamma heavy chain deposition disease" for the process.