Background The life sentence population is growing older and increasing in number. Despite the potential negative physical and social environment in prisons little is known about the prevalence or aetiology of depression in elderly ‘lifers’. Aims To determine the prevalence and associated risk factors of depression in elderly life sentence prisoners. Method One hundred and twenty-one elderly life/indeterminate sentence prisoners from two category B prisons in the United Kingdom were interviewed using the Geriatric Depression Scale and the relationship with prison and non prison specific variables analysed. Results Over half of the prisoners scored above the threshold for mild depression. The length of sentence served and other prison related variables were not associated with the depression score. However, the imported chronic physical ill health was strongly related to depression score. Conclusions Depression in long term prisoners is common and is related to the burden of imported chronic ill health as opposed to specific effects of imprisonment.