Spontaneous proteinuria in otherwise clinically normal adult Beagles 4-6 years old was studied for 2 years. Eighteen dogs, representing a population of 218 Beagles, were placed into three groups: group I, nonproteinuric; group II, intermittently proteinuric; group III, persistently proteinuric. The groups were alike on the basis of laboratory tests, except urinary protein loss. Proteinuria was persistent in most affected dogs but not progressive during the 2 years. The loss of proteins with high molecular weight, including alpha-, beta-, and gamma-globulins, suggested the proteinuria was of glomerular origin. There were glomerular lesions but no other significant change in the kidneys and urogenital system. Lesions were generalized and characterized by prominent, local or diffuse mesangial proliferation and by thickening, wrinkling, and splitting of the glomerular basement membrane. The subendothelial space was often widened and contained electron-dense deposits. Similar electron-dense deposits, as well as lipid and mineral, were in the mesangium. Alterations in visceral epithelial cells and endothelium were prominent. Periglomerular sclerosis was present but tended not to correlate with the severity of mesangial change in any given renal corpuscle. The severity of both mesangial and periglomerular changes increased with increasing proteinuria. Immunofluoescence studies demonstrated granular discontinuous localization of IgG and betaIC-globulins in the glomerular capillaries and mesangium. Similar localization was seen but to a lesser extent in nonproteinuric dogs. The glomerular lesions seen in these clinically healthy, proteinuric dogs are similar to those described in various canind diseases associated with terminal renal failure.