Summary While the FEV1 had been recognized as an excellent indicator of disability, it is not very sensitive to early and mild disease. In cystic fibrosis (CF) small airway disease is believed to be one of the early hallmarks and indices such as the FEF25–75 and FEF75 have been proposed as sensitive markers of early disease. The site of early disease in asthma is not as well worked out. Recently a study of more than 20,000 spirometries found that neither of these indices added anything to the FEV1/FVC but that study was not disease specific and contained both adults and children and the adults were the most numerous. To see if this would be true in children, 1,175 spirograms from children 6 to 18 years of age with CF or asthma whose FEV1 and FVC were above the lower limit of normal were ...