In order to evaluate clinically and spirometrically the efficacy and safety of clenbuterol in comparison with terbutaline and placebo, 30 pediatric patients, in three groups of ten subjects each one, were studied. The study was designed single blind with control group (placebo), randomized, prospective and observational. Total average age was 8.2 years, weight 29 kg and size 130 cm. Nineteen subjects were male and 11 female. Group I received clenbuterol (0.0012 mg/kg/day) solution bid during one week; group II received terbutaline (0.075 mg/kg/day) solution tid and group III, placebo. There were differences statistically significant in the group treated with clenbuterol in FEV1, PMF and VC. Side effects were without importance.