Naturally occurring collagenofibrotic glomerulonephropathy was found in an intact male cynomolgus monkey at 4 years and 5 months of age. Clinical examination showed mild proteinuria and hypoproteinemia, and the kidneys were macroscopically pale and granular on the surface. Histopathology revealed diffuse panglomerular swelling with the accumulation of eosinophilic amorphous material that was negative for amyloid staining and periodic acid-Schiff, but positive for collagen fibers by Mallory-azan staining in the glomerular capillary wall and mesangial area. Immunohistochemistry revealed a strong positive reaction with anti-type III collagen antibody in the glomeruli. The mesangial area was widened, but there was no increase in the number of mesangial cells. No lesion was observed in the efferent/afferent arterioles or other arteries. Mild tubular degeneration was observed in the cortex, occasionally with cholesterin crystals and interstitial lymphocytic infiltration. Electron microscopy revealed various-sized collagen fibrils with typical banding periodicity in the subendothelial space and the mesangial area. The present case offers information on spontaneous and life-threatening primary glomerular fibrosis in a young monkey suggesting an idiopathic familial disease similar to those observed in humans.