Background: FEV¡ is not easily obtained in preschool children. Objective: To evaluate the utility ofFEV0¡5 vs FEV¡ in the spirometry of asthmatic preschool children. Material and Methods: spirometry was performed to 39 asthmatic children and 77 controls. Each child performed maneuvers during 15 minutes without nose clip and if required a computer-animation program was used. An acceptable curve was defined as a register with evident peak expiratory flow (PEF) without sudden cessation of air flow at more than 20%> of the previously measured PEF. Results: 94%> and 90% of healthy and asthmatics performed at least two acceptable curves. Median age in healthy children was 4.7 years-old (2.5 to 5.9) and 3.8 years-old (2.3 to 5.2) in asthmatics. FEV¡ was obtained in 51.3% of controls and in 43% of asthmatics. In contrast FEV0.S was obtained in all the children. A significant bronchodilator response was observed in FEV0.S in 49%> of asthmatics. Conclusions: FEVo.5 was more useful than FEV¡ in interpreting spirometry in 3 to 5 years old children with asthma