OBJECTIVES: To describe common barriers that limit the effect of guidelines on patient care, with emphasis on recommendations for triage in the Agency for Health Care Policy and Research (AHCPR) Unstable Angina Clinical Practice Guideline. DATA SOURCES: Previously reported results from a prospective clinical study of 10,785 patients presenting to the emergency department (ED) with symptoms suggestive of acute cardiac ischemia. STUDY DESIGN: Design is an analysis of the AHCPR guideline with regard to recognized barriers in guideline implementation. Presentation of hypothetical scenarios to ED physicians was used to determine interrater reliability in applying the guideline to assess risk and to make triage decisions. PRINCIPAL FINDINGS: The AHCPR guideline's triage recommendations demonstrate (1) poor interobserver reliability in interpretation by ED physicians; (2) limited applicability of recommendations for outpatient management (applies to 6 percent of patients presenting to the ED with unstable angina); (3) incomplete specifications of exceptions that may require deviation from guideline recommendations; (4) unexpected effects on medical care by significantly increasing the demand for limited intensive care beds; and (5) unknown effects on patient outcomes. In addition, analysis of the guideline highlights the need to address organizational barriers, such as administrative policies that conflict with guideline recommendations and the need to adapt the guideline to conform to local systems of care. CONCLUSIONS: Careful analysis of guideline attributes, projected effect on medical care, and organizational factors reveal several barriers to successful guideline implementation that should be addressed in the design of future guideline-based interventions.