To enhance comparability in reports on survival from out-of-hospital cardiac arrest, an international task force recently developed a set of guidelines for uniform terminology, definitions, and data collection for outcome research on cardiac arrest--the Utstein style. Because the data collection recommended is limited to information available through emergency medical services systems, the potential for bias in comparisons of cardiac arrest outcomes remains. By expanding data collection to include the identification of all cases of cardiac arrest in the community, including patients who do not present for care by an emergency medical services system, a population-based approach can be achieved. We review the strengths and limitations of both emergency medical services-based and population-based data collection to assess outcomes of cardiac arrest, outline practical steps required to implement a population-based approach, and suggest that extension of the Utstein style guidelines to include all cardiac arrest cases within a defined population is needed to minimize potential bias in comparisons of cardiac arrest outcomes across communities or over time.