Introduction: The assessment of respiratory function in preschool children, which has recently been attracting considerable interest, has several methodological particularities. Whether this is feasible in clinical practice with large groups of patients still needs to be investigated. Aim: To assess the feasibility of pulmonary function testing in preschool children in clinical practice, and report the degree of success achieved according to age. Methods: Retrospective analysis of lung function tests performed in children from 2 to 6 years old at the respiratory function laboratory of CUF Descobertas Hospital between September 2006 and August 2011. Whole-body pletismography without occlusion for specific airway resistance (sRaw) assessment and animated spirometry were performed using the equipment Jaeger 4.65 (Viasys Healthcare), before and after 400 mu g of inhaled salbutamol via a spacer device. The research fulfilled international criteria (ATS/ERS) for acceptability and reproducibility. Results: Of 1,239 lung function tests performed, 1,092 (88\%) had acceptable and reproducible criteria for spirometry (children with a mean age of 4.3 +/- 0.91 years; 60.7\% male), and 979 (79\%) for sRaw measurement. We were able to report FEV1 in 801 (65\%) tests (children with a mean age of 4.5 +/- 0.89 years). In 23 (2\%) tests it was only possible to report FEV0.5 (children with a mean age of 3.5 +/- 0.67 years) and in 268 (22\%) only FEV0.75 (children with a mean age of 4.0 +/- 0.89 years). Conclusion: Spirometry and sRaw assessment in preschool children can be used in clinical practice, with an increasing success rate as children get older. Reporting maneuvers of 0.5 or 0.75 seconds facilitates spirometric evaluation in a larger number of children. (C) 2012 Sociedade Portuguesa de Pneumologia. Published by Elsevier Espana, S.L. All rights reserved.