There is a wide variability in clinical practice for treating acute asthma (AA) in the emergency department (ED), interfering in the quality of care. The purpose of this study was to evaluate the impact of a clinical pathway on the management of AA in the ED. We conducted a prospective before-after study of patients presenting with AA to the adult ED, during five separate periods (from January to March): in 2001 (pre-protocol group), 2002, 2003, 2004, and 2005 (6 months without educational reinforcement). We evaluated the effects of the recommendations on objective assessment of severity, diagnostic resource utilization, use of recommended and non-recommended therapy, and outcomes. The 2001, 2002, 2003, 2004, and 2005 groups comprised, respectively: 108, 96, 97, 98, and 101 patients. There was a significant increase in the use of pulse oximetry (8.3%, 77.1%, 88.7%, 95.9%, and 97.0%, respectively; p < 0.001). There was an increase in the use of peak expiratory flow rate from 2001 to 2004 (4.6%, 20.8%, 28.9%, and 48.0%) and a decrease after a period without educational efforts (29.7%, p < 0.001). Although the overall use of systemic corticosteroids was not changed, there was a significant increase in the use of oral steroids (p < 0.001). There was a decrease in aminophylline utilization (p = 0.005). Length of stay in the ED was significantly reduced (p = 0.04). There was no effect on hospital admission or emergency discharge (p = 0.193). The AA clinical pathway applied in the ED was associated with a positive effect on improving the quality of care.