Asthmatic crisis is one of the most frequent urgencies in children. One of the most difficult decision for a physician is to determine what patient requires only ambulatory treatment and what patient needs hospital treatment. Many investigators have developed predictive scores, but to date conclusions are confusing. The purpose of the present study was to compare the value of arterial oxygen saturation versus peak expiratory flow rate measurements as predictors of the outcome of the asthmatic crisis in children. We performed a prospective, longitudinal, observational, comparative study in 50 children (30 males, 20 female), with ages between 6 and 18 years old, with the presence of a moderate asthmatic crisis. We measured arterial oxygen saturation and peak expiratory flow rate at the moment of their arrival to urgencies and at 30 minutes, 2, 4 and 24 hours after the administration of nebulized albuterol (100 mcg/k/dose). We found significant differences on baseline arterial oxygen saturation and the registrations taken at 2, 4, and 24 hours posttreatment (0.01), and we could only find significant differences between baseline peak expiratory flow rate and the registrations taken at 24 hours pretreatment. In conclusion arterial oxygen saturation is a better predictor than peak expiratory flow rate in the evolution of the asthmatic crisis in children.