BACKGROUND: The mortality of people suffering from psychiatric illnesses is far higher than that of the general population, all categories of diagnosis combined; mortality statistics can be used as an index of quality of care. The aim of this study was to assess the all-cause mortality in psychiatric patients covering all diagnostic groups. METHODS: The living or deceased status of 4,417 patients of majority age hospitalised in a public mental health establishment between 2004 and 2007 were requested from French National Institute for Statistics and Economic Studies on 1st January 2011. The cause of death of those people who had died was obtained from French National Institute for Health and Medical Research and comparative standardized mortality ratios (SMR) were established from the population in a region of northern France of the same age in 2006. RESULTS: The study population was made up of 54% men and 46% women, median age 41 and 45 years old, respectively. Four hundred and seventy-three people died during the period studied. The SMR were 421 for men (95% CI 378-470) and 330 for women (95% CI 281-388). The highest SMRs were found in patients aged 35-54, with a 20-time higher mortality risk than the general population of the same age. CONCLUSION: Our study confirms the considerably higher mortality in psychiatric patients than in general population, particularly in mean age and mostly due to an unnatural cause.