A fourteen-year-old Persian cat was referred because of poorly controlled diabetes mellitusdespite insulin and dietary treatment. Clinical signs were severe polydipsia/polyuria (pupd),poor hair coat quality, stomatitis and hind limb weakness. At the time of initial presentation, hewas treated with glargine insulin (0,75 IU/kg BID). A low dose dexamethasone suppression test(LDDST) revealed hypercortisolism (HC). The cat was additionally treated with trilostane, andremission of diabetes mellitus was obtained one year later.This case illustrates the importance of diagnosing an underlying cause of poorly controlleddiabetes mellitus. Although hypercortisolism is rare in cats, it is important to consider the diseasein these cases. The hypercortisolism in this cat was efficiently managed with trilostane, resultingin a good quality of life.