We evaluated 174 children with acute asthma and/or wheezing attending two different settings, the allergy clinic (AC) and the emergency room (ER), and compared clinical symptoms and spirometric findings with arterial oxygen saturation as measured by pulse oximetry (SpO2). Seventy-four children (4 months to 15 years old) seen at the AC and 100 children (2 months to 14 years old) seen at the ER for the treatment of acute asthma and/or wheezing were evaluated and a clinical score was attributed on the basis of their symptoms. In addition, the heart rate (HR) was recorded and SpO2 was measured. Among the children seen at the AC, 58 were able to perform pulmonary function tests, and the forced respiratory volume in 1 sec (FEV1) and forced expiratory flow between 25% and 75% of the forced vital capacity (FEF(25-75)) were determined. Children from both groups underwent treatment with a nebulized beta2-agonist (Fenoterol 0.5% solution, 0.08 mg/kg/dose, maximum 2.5 mg) and were re-evaluated after 30 min. Our results showed a significant correlation between decrease in clinical scores and increase of SpO2 following treatment with bronchodilator in both groups of children. SpO2 levels correlated positively with FEV1 and FEF(25-75) values, and negatively with clinical scores and heart rate. The data revealed that a clinical score greater than 3 and an SpO2 < 94% were associated with increased severity of the asthma attack. In addition, SpO2 levels < or = 92% were associated with a 6.3-fold greater relative risk for requiring additional treatment. We concluded that determination of oxygen saturation by pulse oximetry is helpful in monitoring the severity of an acute exacerbation of asthma and/or wheezing, and has a prognostic value.