Objective. Constipation is a prevalent problem in patients with psychiatric disorders; it reduces quality of life and may lead to severe complications. The prevalence distribution of constipation across all psychiatric diagnoses in patients with severe mental illness (SMI) has hardly been studied. The aim of this study is to estimate the association between psychiatric disorders and constipation in SMI inpatients. Methods. The strength of the association between constipation (based on use of laxatives) and DSM-IV psychiatric diagnosis was studied in a cross-sectional study with "adjustment disorders" as the reference group. The association was analyzed using logistic regression. Results. Of the 4728 patients, 20.3% had constipation. In the stratum of patients older than 60 years, all psychiatric categories except for substance related disorders were significantly associated with a higher prevalence of constipation (odds ratios ranging from 3.38 to 6.52), whereas no significant associations were found in the stratum of patients between 18 and 60 years (odds ratios ranging from 1.00 to 2.03). Conclusion. In the elderly, all measured psychiatric diagnoses are strongly associated with an increased prevalence of constipation. Physicians should be extra alert for constipation in SMI patients, independent of specific psychiatric diagnoses.