The strain of FGS/Nga mouse is reported to develop proteinuria and progressive glomerulosclerosis. We studied the renal pathology of that strain periodically for 1 year. Focal and segmental glomerulosclerosis was observed 3 months after birth and the lesion progressed to the glomerular obsolescence in a year. Electron microscopic study revealed electron dense deposits (DD) in the mesangium and the splitting of glomerular basement membrane. Studies using immunofluorescence and immunoelectron microscopy revealed that these DD were contained IgA, IgM, C3 and the retroviral envelope antigen (gp70). Clinically, proteinuria began at the age of 3 months and the renal function was decreased on time course. No other organs were involved. We studied the renal lesions of FGS mice by the histological and immunohistochemical methods and concluded that this mouse strain provides the tool for studying the mechanisms of the progression of glomerulosclerosis.