In order to investigate the relationship between the fatty acid composition of the dogs' tissue and dementia, we compared the fatty acid composition of red blood cells (RBCs) between 15 mildly demented dogs and 15 normal controls (≥10 years old). In the mildly demented dogs polyunsaturated fatty acids (PUFA) were significantly decreased and the ratios of arachidonic acid to total PUFA (AA⁄PUFA) were signiflcantly increased compared with normal dogs. AA⁄PUFA ratios had a significant correlation with dementia scores of 30 dogs (r=0.48). Dog food analysis showed that the average n-6 fatty acids⁄n-3 fatty acids ratios, a parameter influencing AA⁄PUFA ratios in tissues, were very high in pellet-type dog foods (n=20, the ratios=12). In conclusion, high AA⁄PUFA values in RBCs might be a risk factor of mild dementia in dogs. It seems not prudent to provide AA supplements to mildly demented dogs.