In the course of a year the prevalence of psychiatric comorbidity as well as the effect of psychiatric comorbidity on the length of stay on two internal medical wards we investigated. When examined by separate methods, the prevalence of psychiatric comorbidity reached 36 % (ICD-10 diagnosis) in a sample of 511 medical inpatients of a university hospital. The study shows that patients with psychiatric comorbidity have a significantly longer length of stay - up to 8.2 days compared with inpatients with mere internal diagnosis. This association was not influenced by the length and the severity of the illness. The findings lead to the conclusion that psychiatric comorbidity is a central contributor to one of the most important factors of medical costs - the length of stay.