Non-bacterial thrombotic endocarditis or marantic endocarditis is an uncommon and frequently terminal condition. We report non-bacterial thrombotic endocarditis presenting as a psychotic illness. The underlying cause was found to be adenocarcinoma of the lung. The patient was nursed on a specialist joint medical/psychiatric unit. Many medical illnesses can present with psychosis in older people: this is the first case report of this presentation in non-bacterial thrombotic endocarditis.