Introduction In paraproteinemia, especially in multiple myeloma, the origin and morphology of lesions in the kidneys still remain controversial. In many studies mostly of tissue obtained post mortem and examined with light microscopy, a discrepancy between impairment of renal function and minimal pathomorphological findings is often striking. The few studies done with the electron microscope up to now present a wide variety of morphological findings probably due to the different stages of kidney lesions examined, generally in patients with renal insufficiency and nephrotic syndrome. We have therefore tested the question whether early lesions are detectable in the nephron of patients with paraproteinemia who do not show any impairment of renal function. Material and methods Kidney biopsies were obtained from 8 patients with an average age of 58 years. 6 had multiple myeloma (3 IgG-, 1 IgA-, 2 Bence-Jones plasmacytoma), 1 Waldenström's disease and 1 an idiopathic paraproteinemia. Confirmation of the diagnoses was obtained by immunoelectrophoresis, bone marrow biopsy, and sternal puncture. In all cases creatinine clearance was normal and proteinuria was not increased substantially even in patients with a slight Bence-Jones-proteinuria. For electron microscopy small pieces from the renal biopsies were fixed in aldehydes followed by osmium tetroxide and embedded in Epon. The thin sections were stained with uranyl acetate followed by lead citrate. Results and discussion Under the light microscope plastic-embedded semithin sections show only a discrete enlargement of the mesangium in the glomeruli, but never alterations commonly associated with the so-called “myeloma kidney”. Electron microscopy confirmed this mesangial enlargement displaying deposits of fibrillar material, predominantly microfibrils extending to the basement membrane of the glomerular capillary loops and causing an irregular thickening of the lamina rara interna. These fibrils are sometimes of a slightly variable structure but lack periods, whereas the other layers of the basement membrane and the endothelial cells do not show any lesions. In our opinion these striking deposits of microfibrils are not an only nonspecific alteration, i.e., mesangium-matrix-like material due to an increased permeation of low molecular protein, but at least partially possible precursors of amyloid. Experimental findings demonstrate that some of the amyloid fibrils are deposited first at this site of the glomeruli, and the very close structural relation between Bence-Jones-protein and amyloid fibrils are in support of this hypothesis. Only in one case were typical amyloid deposits observed in the pericapillary area of the outer medullary zone. The proximal tubules have an increased number of micropinocytotic vesicles, large reabsorption vacuoles and dense bodies in the apical zone of their cells and an extensive smooth endoplasmic reticulum. The lesions in the proximal tubular cells seem to be in agreement with an increased protein reabsorption in paraproteinemia. Deposits of coarse fibrils with a definite period are conspicuous. They are arranged predominantly in an elongated or circular way in the basement membrane of the proximal tubules and sometimes in the glomerular capillaries and mesangium. These unique coarse fibrils with a definite period are suggestive of large aggregates of immunoglobulin fragments.