Despite the emphasis on appropriateness of emergency department utilization, there is currently no methodology for assessing appropriateness nor measures for a community to evaluate and improve effectiveness of its E.M.S. system in treating emergencies at the appropriate emergency room and nonemergencies at appropriate alternative settings. The writer feels that the one current strategy which is widely accepted, the A.M.A.'s concept of categorization, has substantial methodologic limitations. The present paper suggests a methodology and a set of measures to assess appropriateness of utilization and illustrates them with data from Buffalo, New York, on a chart review of 24,594 emergency department patient medical records and interviews with 888 patients and their attending physicians. The measures include: 1) distribution of ambulance and critically ill patients by A.M.A. category of hospital, 2) utilization, characterized as system under- or over-response or as appropriate utilization, and 3) physician judgments as to indicated alternative treatment sites, which are suggested as evaluative tools to hospital and communities for E.M.S. assessment, and to federal and state agencies as performance standards, for project selection for funding and monitoring for outcome evaluation.