An 81-year-old woman was hospitalised for behavioural disorders that had been progressively emerging over a period of few weeks. The symptoms appeared to worsen over time. A diagnosis of vascular dementia, complicated by psychosis, was initially hypothesised. The inefficacy of the antipsychotic/benzodiazepine medications used, along with the presence of hypertension, hypokalaemia and metabolic alkalosis (resistant to pharmacological attempts of correction), as well as the hirsutism and the development of several infections, led us to consider Cushing's syndrome. Endocrinological analysis suggested ectopic adrenocorticotropic hormone (ACTH) secretion. Although endogenous Cushing's syndrome is rare in older people, it should always be considered among the differential diagnosis of behavioural disorders.