The authors critically reviewed 26 international and American outcome studies that assessed the effect of psychiatric comorbidity on length of stay for medical/surgical inpatients. Three generations of American studies were defined according to the rigorousness of the methodology. Eighty-nine percent of all studies with sample sizes greater than 110 and 75% of the prospective, rigorously controlled, American studies found a significant association between psychiatric comorbidity and increased length of stay. The findings lead to the conclusion that impaired cognition associated with delirium and dementia, depressed mood, and other personality variables contributes to prolonged hospital stays and greater utilization of hospitals and other health resources after discharge. Recommendations for future research are suggested.