Fructosamines are glycated serum proteins that, depending on their life span, reflect glycemic control over the previous 2 to 3 weeks. The nitroblue tetrazolium reduction method adapted to auto analysis appeared to be a practical means to assay fructosamine quickly, economically, and accurately. The upper limit of the reference range is 374 mumol/L in dogs (95% percentile) and 340 mumol/L in cats (95% percentile). Newly diagnosed diabetic dogs and cats that had not undergone previous insulin therapy had significantly higher fructosamine concentrations than nondiabetic animals. In diabetic dogs that were receiving insulin therapy, the fructosamine test reflected the glycemic state far more accurately than did individual blood glucose measurements. Animals with satisfactory metabolic control revealed fructosamine concentrations within the reference range, whereas fructosamine concentrations above 400 mumol/L indicated insufficient metabolic control. On the basis of fructosamine concentrations, cats with a transitory hyperglycemia and cats with diabetes mellitus were differentiated. The fructosamine test is a valuable parameter for the diagnosis and metabolic control of diabetes mellitus in dogs and cats.