OBJECTIVES: To investigate the factors influencing the Severity of illness index of the All Patient Refined DRGs (APR-DRG) classification of patients experiencing an ischemic stroke. METHODS: We conducted a retrospective analysis of ischemic stroke patients classified as “APR-DRG 045: CVA & Precerebral Occlusion with Infarct” between 2005-2007 admitted to the leading teaching hospital in Belgium. Each admission was assigned a primary diagnosis, followed by one or more secondary diagnoses. Based on an algorithm combining these diagnoses a SOI level was assigned to each hospitalization, informing the payment/reimbursement for each admission. This classification allows for the relative comparison of patient subgroups within each APR-DRG and severity subclass, and was designed to reflect the relative resources required for treatment, enabling for the casemix adjustment of the payment/reimbursement system. RESULTS: