A 62-year-old man with a syndrome of gradually progressive cognitive deterioration accompanied by paranoid features is described. He had been taking quinidine since 1974 for a recurring supraventricular tachyarrhythmia. Examination revealed a suspicious man with widespread patchy cognitive deficits but no focal neurological signs. An exhaustive range of investigations gave unremarkable results. Within 24 hours of cessation of quinidine there was a dramatic improvement in his mental state and, after a further four days, he had returned essentially to normal with no demonstrable cognitive deficits. Several months later he suffered a recrudescence with prominent paranoid and depressive features which gradually settled after commencement of pimozide and dothiepin. It is likely that these events reflect a recurring functional psychosis which was precipitated or exacerbated in the first instance by quinidine. This represents a significant complication of quinidine therapy which has been largely unrecognized. Therapy with quinidine should be considered as a potential contributing factor in any patient with dementia or a functional psychosis who is also taking this drug.