Comorbidity measures are assigned using the AHRQ comorbidity software. The AHRQ comorbidity measures identify coexisting medical conditions that are not directly related to the principal diagnosis, or the main reason for admission, and are likely to have originated prior to the hospital stay. Comorbidities are identified using ICD-9-CM diagnoses and the Diagnosis Related Group (DRG) in effect on the discharge date. The prefix "CM_" has been added to the AHRQ comorbidity software data element names to distinguish the comorbidity measures from other HCUP data elements. For more information, please refer to the materials available on the Tools and Software page of the HCUP User Support Website (http://www.hcup-us.ahrq.gov/tools_software.jsp).