There is no consensus about reproducibility and reliability of spirometry in young children. We evaluated forced expiratory maneuvers from 98 children aged 3 to 5 years with a variety of respiratory disorders before and after bronchodilator treatment. Forced vital capacity (FVC) and forced expiratory volume in 1 sec (FEV1) were analyzed for reproducibility by the American Thoracic Society criteria and for reliability based on the coefficient of variation (CV%). Over 90% of the patients cooperated, however, while 95% could exhale for at least 1 second, very few generated an FEV1 on all 6 "best" efforts. This clearly improved with age. Of all patients nearly 60% performed reproducible pre- and postbronchodilator sets of FVC but only 32% performed reproducible sets of FEV1. Based on the CV%, those patients who could reproducibly perform an FVC and FEV1 did it quite reliably (mean CV%, 9.38 and 7.01 for FVC and FEV1, respectively). We conclude that while some very young children can perform spirometry, reliability of performance cannot be assumed in this age group.