Collagenofibrotic glomerulopathy is a rare glomerular disease characterized by the massive deposition of type III collagen in mesangial and subendothelial spaces. We observed markedly increased serum hyaluronan levels in patients with collagenofibrotic glomerulopathy; levels in three patients were more than 1,000-times greater than the normal upper limit. However, one kidney transplant patient had normal serum hyaluronan levels. We found that serum levels and activities of the enzyme hyaluronidase were normal, and hyaluronan was not markedly deposited in the mesangial or subendothelial spaces. Our findings suggest that serum hyaluronan levels may be a specific diagnostic marker of collagenofibrotic glomerulopathy, and kidney transplantation may alleviate marked increases in serum hyaluronan.