A case of acute paranoia in a 64-year-old woman with a previous history of one depressive episode is described. Accurate diagnosis was hampered by her illogical and tangential answers to questions and the development of an organic brain syndrome while on neuroleptics and anticholinergics. She did not tolerate tricyclic antidepressants and was unresponsive to three electroconvulsive therapy (ECT) treatments. Because of the previous history of depression and the presence of some depressive symptoms, the possibility of a depression underlying her paranoid psychosis was considered. A dexamethasone suppression test (DST) was performed to confirm this and was found to be abnormal. ECT was then continued with the eventual result of complete remission in symptoms. The authors discuss the potential usefulness of the DST in confirming a diagnosis of depression in selected cases of late onset psychosis.