A five-year-old intact male rottweiler was presented with a history of episodic weakness and mild-generalised seizures. A tentative diagnosis of an insulin-secreting tumour in the pancreas was made based on fasting hypoglycaemia with concomitant hyperinsulinaemia and a subnormal fructosamine value. The diagnosis was confirmed by exploratory coeliotomy, intravenous infusion of methylene blue, histopathology and immuno-histochemical analysis of suspected neoplastic tissue. Fructosamine assays are traditionally used for monitoring the metabolic status of diabetics where a single elevated measurement reflects persistent hyperglycaemia. This report suggests that a single low measurement of fructosamine may indicate persistent hypoglycaemia and may be helpful, in conjunction with an insulin measurement, in the diagnosis of insulin-secreting tumours.