Measurements of serum fructosamine and glycated haemoglobin are increasingly used to complement plasma glucose concentration in the fasting dog to diagnose diabetes mellitus and to monitor the response to treatment. These measurements are not affected by acute changes in the glucose concentration and reflect the average plasma glucose concentration over the preceding one to two weeks in the case of serum fructosamine and two to three months in the case of glycated haemoglobin. Both components can be measured in canine blood samples, but glycated haemoglobin is still not measured routinely; however, the serum fructosamine concentration can be measured accurately by means of simple spectrophotometric assays. The sensitivity and specificity of serum fructosamine in diagnosing diabetes mellitus in dogs with clinical signs of the disease are very high (0.93 and 0.95, respectively). Furthermore, serum fructosamine can be used as a reliable screening test to identify diabetic dogs in an average middle-aged to older hospital population. In addition, serum fructosamine can distinguish between hyperglycaemic non-diabetic dogs and hyperglycaemic diabetic dogs. Preliminary data suggest that therapy can be safely monitored and regulated on the basis of serial measurements of the serum fructosamine concentration in diabetic dogs.