BACKGROUND This article provides evidence about the relationship between psychiatric disorders, physical disorders and hospital use in the general medical sector using a broadly based survey of the US population. METHODS The data are from the 1989 National Health Interview Survey. This survey contains medical and mental health evaluations for the entire sample. In a multivariate framework, the author estimates the effect of mental illness on the probability of being admitted to a general hospital, the number of admissions and the length of stay. RESULTS Hospital use in the general medical sector is significantly higher for persons with coexisting physical and psychiatric conditions than for those with no psychiatric disorders. For a wide range of medical conditions, the predicted number of hospital admissions and the length of a hospital stay increase substantially when the physical illness is accompanied by a psychiatric condition. CONCLUSIONS One implication of this finding is that economic evaluations of alternative psychiatric treatments should consider any differences in hospital costs related to the treatment of coexisting medical conditions. Another implication pertains to health care systems where insurers have some discretion over which individuals to insure. In the absence of adequate adjustments in insurance payments for high-risk potential enrollees, psychiatrically disabled persons may have more limited access to health insurance.