Medical inpatients often suffer from comorbid psychiatric illness, which has been shown in previous studies to be associated with longer hospital stays. The present analysis used a large representative dataset to examine the impact of patient demographic and clinical characteristics on the relationship between psychiatric comorbidity and hospital length of stay. Analyses showed the existence of a psychiatric comorbidity predicted longer hospital stays for medical inpatients. However, in comparison to previous research, this effect was attenuated in this sample. Patients with mental disorders who were elderly, on Medicare, and those with schizophrenia or mood disorders were especially at risk for slightly longer lengths of stay.