While xanthogranulomatous pyelonephritis in humans tends to be rich in birefringent fat, this variety cannot be demonstrated microscopically in the feline morphological counterpart of the condition, which is, however, rich in readily stainable fat. Since normal feline kidney is rich in such lipid, a qualitative and quantitative comparison of the lipid in human, feline and canine kidney was carried out in an attempt to throw further light on the possible origin of the birefringent fat in the human disease. No significant difference could be found in the amount of cholesterol in the 3 species. Despite its visual prominence in feline kidneys, human kidney was richer in neutral fat and the percentage of total lipid formed by cholesterol was greater in humans than in the other species. The results suggested that cases of xanthogranulomatous pyelonephritis may occur in those kidneys unusually rich in cholesterol.