Massive deposition of collagen fibrils was found in the renal glomeruli of two pigs. The affected pigs were a 6-month-old female hybrid pig with systemic mycobacteriosis and a retired sow showing azotemia. Microscopically, various amounts of a homogeneous eosinophilic substance were deposited within the expanded mesangium of almost all renal glomeruli in both cases. The deposit was also seen occasionally on the glomerular capillary walls in the sow. Capillary lumens were occluded in the glomerular segments with heavy deposition. Obsolescent glomeruli were often surrounded by lymphoid cells. The deposited substance was Congo red negative and stained deep blue with Heidenhain's azan staining. Immunohistochemical evaluation revealed that the major component of the homogeneous substance was type-III collagen. Electron microscopic examination showed that the deposits were masses of fibrils of variable length, width, and shape. The fibrils had transverse bands with a periodicity of approximately 60 nm, consistent with collagen fibrils. These glomerular changes were identical to those of collagenofibrotic glomerulonephropathy that has recently been described as a new glomerular disease in humans. The glomerular lesion may be the result of unusual production of type-III collagen by mesangial cells.