Percutaneous renal biopsies from 4 patients with clinically unsuspected kappa light chain nephropathy were studied using light, immunofluorescence, and electron microscopy. The diagnosis in each case was established by demonstrating monoclonal kappa light chain deposits in basement membranes and basement membrane-like structures of glomeruli, tubules, and blood vessels by immunofluorescence microscopy. Characteristic electron dense deposits occurred in every case but the intensity and distribution of electron densities did not correlate with the immunofluorescence findings. When light chain aggregation occurred, as evidenced by the distribution of electron dense deposits, it was proportional to the amount of basement membrane-like material as if these immunoglobulins had a particular affinity for structures chemically related to basement membranes. Although active tubulointerstitial lesions were prominent in all biopsies, there was considerable variation in glomerular pathology with only 1 case exhibiting the typical nodular glomerulosclerosis. Correlation of the light, immunofluorescence, and electron microscopic findings in these cases suggests that the pathogenesis of kappa light chain nephropathy is related to light chain nephrotoxicity directed to basement membrane-like structures with subsequent alterations in hemodynamics and structural renal damage.