Objectives: To investigate natural-cause mortality risk in people with psychiatric admission histories in a national population-based cohort. To estimate risks in relation to psychiatric diagnostic categories and for specific causes of death., Methods: We studied a 21-year cohort (1973-93) of 4.1 million Danish adults with linkage to national psychiatric and mortality registers. Person years were stratified by psychiatric diagnostic category and age-standardized mortality ratios (SMRs) were estimated versus the general population., Results: Risks of death by natural causes were higher across a range of psychiatric illnesses in both sexes. We observed SMRs greater than 200 in men and women with alcoholism, drug abuse, organic psychoses, dementia, and learning difficulties. Alcoholism and drug misuse in particular were important causes of premature mortality. The highest cause-specific SMRs were for nervous system diseases, gastrointestinal diseases, lung diseases, and "all other natural causes"; the lowest were for neoplasm. The greatest excess, in terms of absolute numbers, was for circulatory disease mortality., Conclusion: Adults experiencing a range of psychiatric illnesses are more likely to die at any age, and also prematurely, from natural causes. The consistency of elevated risk across psychiatric diagnoses and causes of death indicates an important health inequality. Those involved in planning and providing mental health services should address the heightened need for physical health care in psychiatric patients.