Glomerular lipidosis is a disease characterized by lipid accumulation in mesangial cells but that has not been fully investigated in avian species. We examined four wild and two laboratory-reared Japanese rock ptarmigans (Lagopus mutus japonicus)—an endangered avian species—presenting vacuolar deposits in the glomeruli. All cases had vacuolar deposits in the glomeruli. In the wild cases, fewer than 30% of all glomeruli were affected, compared with more than 90% in the laboratory-reared cases. In the wild cases, most deposits were mild and restricted to the mesangial areas of glomeruli. In the laboratory-reared cases, nearly all of the deposits covered entire glomeruli. Electron microscopy of mild deposits revealed vacuoles in the cytoplasm of mesangial cells. These vacuoles were positive for Sudan III, Sudan black B, oil red O, Nile blue, periodic acid-Schiff, Schultz test, and digitonin stain and were negative for performaric acid-Schiff stains. Based on these results, we diagnosed the glomerular lesion as glomerular lipidosis caused by uptake of low-density lipoprotein in mesangial cells. Except for one wild case, all cases exhibited renal tubular oxalosis. The severity of tubular oxalosis tended to be related to the severity of glomerular lipidosis: In cases of mild glomerular lipidosis, tubular oxalosis was also mild or absent. We therefore diagnosed the primary lesion as glomerular lipidosis accompanied by tubular oxalosis. The four wild cases came from different zones and therefore had no opportunities to interbreed and no common relatives. We believe these data support the hypothesis that glomerular lipidosis is a disease of the general population of Japanese rock ptarmigans. This is the first report of glomerular lipidosis accompanied by renal tubular oxalosis in an avian species.