The dexamethasone suppression test (DST) was administered to 95 patients referred to a psychogeriatric assessment service. Non-suppression of plasma cortisol was found in 28 out of 48 patients (58%) with senile dementia and all patients with arteriosclerotic dementia or acute confusional states. Non-suppression could not be explained by associated depressive symptoms. The DST was confirmed as a valid diagnostic test for endogenous depression in the elderly, but its value in distinguishing true dementing illnesses from depressive pseudodementia was not supported. The clinical implications of these findings for interpreting DST results in the elderly are discussed.