ObjectivePrevious research has shown that hospital length of stay among medical patients is significantly increased by comorbid mental illness, in particular depression. However, few studies have examined the length of stay effect of comorbid physical and mental illness among psychiatric patients.MethodThe present study examined the effect of comorbid physical and psychiatric illness on hospital length of stay among 2323 psychiatric inpatient admissions over a 5-year period. Patients were grouped into seven diagnostic categories.ResultsAverage length of stay was significantly longer for patients with comorbid physical diagnoses (mean = 20.01 days) than for patients with no physical diagnoses (mean = 16.63 days). Analyses of the psychiatric categories revealed that the average length of stay for depressed patients was significantly greater for those with comorbid physical diagnoses (mean = 19.73 days) than for depressed patients with no comorbid physical diagnoses (mean = 13.96 days). No other psychiatric group evidenced a significant increase in length of stay for comorbid physical illness.ConclusionsResults suggest that comorbid physical diagnosis increases length of stay among psychiatric patients overall, with increased hospitalization stay for depressed patients, in particular.