Partial (PEFV) and maximal (MEFV) expiratory flow-volume curves were obtained on 47 healthy children, 3 to 13 yr of age, while they breathed air or a helium-oxygen (He-O2) gas mixture. Girls had significantly higher (p less than 0.02) flows at FRC (Vmax FRC) on the air-MEFV curves than on the air-PEFV curves, whereas boys had similar flows on both curves. There was also a significant (p less than 0.04) relationship between the inspiratory volume (before initiating a PEFV curve) and Vmax FRC (on PEFV curves) for the girls but not for the boys. No difference in Vmax FRC (PEFV) and Vmax FRC (MEFV) were observed for either sex when He-O2 was breathed. These observations suggested that girls, with a deep inspiration, can increase their maximal flows, possibly by altering the tone of their airways. In addition, comparison of the air and He-O2 data suggested that girls may tend to dilate preferentially their larger airways with a maximal inspiration. Boys may generally be unable to do this. Such possible sex differences in the mechanical properties of the lung may relate to the different prevalence rates for severe lower respiratory tract illnesses and asthma in the 2 sexes.U