Study objective: To examine the effect of an emergency department program on acute asthma care. Methods: We conducted a before-after study of an acute asthma quality improvement initiative in an urban teaching hospital with 65,000 annual ED visits. in mid-1994, a multidisciplinary group identified deficiencies in acute asthma care, developed and implemented a local Version of the National Asthma Education Program's practice guidelines (including a standard asthma order sheet), and provided new peak flow (PF) meters. The "before" group comprised ail adults with acute asthma seen during January 1994 (n=51); "after" groups comprised all adults with acute asthma seen during October 1994, February 1995, and June 1995 (n=145). Data were compared across months using a nonparametric test for trend. Results: Although patient demographic characteristics and asthma severity were similar across months, ED process of care significantly changed. Initial PF measurements were obtained in 20% of patients before intervention, compared with 82%, 84%, and 83% during the postintervention months (P for trend <.001). Follow-up PF readings were obtained in 22%, 70%, 78%, and 62% (P<.001). Median delays to beta-agonist and steroid therapy decreased by approximately 16 minutes (P<.001) and 34 minutes (P=.04), respectively. Outcomes improved, with median ED length of stay decreasing by 58 minutes (P=.01), and fewer inpatient admissions (P=.05); there was no significant change in 4-week relapse to our hospital. Conclusion: A guideline-based ED asthma program changed clinical practice and improved acute asthma care in a sustained fashion. The effect of this intervention on cost and other outcomes is uncertain.