Our study shows that increased length of stay is associated with the presence of psychiatric comorbidity in inpatients who have certain medical illnesses such as neoplasms; circulatory, digestive, renal, and respiratory disorders; and operative procedures. Our resuits suggest that the effects of psychiatric interventions for such patients should be explored and that studies of the cost-effectiveness of psychiatric interventions should be focused on categories of medical illness in which psychiatric comorbidity is associated with increased length of stay. Collaboration between researchers in medicine and psychiatry is an important approach to containing the cost of hospital care, and by furthering the recognition and treatment of psychiatric comorbidities, such collaboration can also enhance patient's health.