The Charlson index is commonly used for risk adjustment in critical care health services research. However, the literature supporting this methodology has not been thoroughly explored. We systematically reviewed the literature related to administrative database adaptations of the Charlson index. Our review has 3 major findings. First, 2 studies compared Canadian administrative databases with chart review for obtaining Charlson comorbidity data. Agreement between the database and chart review was substantial (kappa > 0.70), and mortality prediction did not differ. Second, 5 database adaptations were identified with the Deyo and Dartmouth-Manitoba adaptations being most popular. Three studies directly compared these 2 popular adaptations and demonstrated substantial agreement (kappa > 0.70) and similar predictive ability for mortality. Third, one study validated the Charlson index for critically ill patients but demonstrated that APACHE (Acute Physiology and Chronic Health Evaluation) II better discriminates inhospital mortality (area under curve 0.67 vs 0.87). Time and cost barriers prevent widespread use of physiology-based risk adjustment in population-based research. The decreased predictive ability of the Charlson index must be weighed against the advantages of using this instrument for population-based research. Future research should focus on updating the Charlson index for recent changes in the prognosis of comorbid diseases and introduction of International Statistical Classification of Diseases, 10th Revision coding of discharge abstracts.