We found a strain of nonobese, nondiabetic (NON) mice which has spontaneous lipid deposition in glomerular capillary lumina. This strain was developed together with a diabetic strain of nonobese diabetic (NOD) mice for the generation of mouse models of diabetes mellitus. In the NON strain, contrary to the name, impaired glucose tolerance (IGT) was observed in about half of the mice. Meanwhile, peculiar glomerular abnormalities which remotely resemble those of diabetic nephropathy were observed in the NON strain. The lesions were characterized by massive lipid accumulation with proteinaceous material within the glomerular capillary lumina. In addition, positive staining for immunoglobulins, especially IgM, was observed by immunofluorescence microscopy. The overall frequency of this lesion was 91%. Mesangiolysis, capillary ballooning with many small lipid vesicles were the striking features by electron microscopy. Histochemical analysis revealed the presence of various lipids in these lesions. However, as far as we examined, these lesions did not correlate with hyperlipidemia or IGT. Lymphoid follicle-like structures were seen around the renal arterioles. The cellular components of these lymphoid follicles reacted with monoclonal antibodies to L3T4. High levels of serum immunoglobulins were observed in this strain. We suppose that the immunological disorders may have some bearing in the evolution of this lesion in NON mice. We believe that this model may be of use in studying the role of lipid derangements in renal diseases.