The use of spirometry in the assessment of children with asthma is taking on new importance with the realization that considerable airway obstruction may exist in the absence of clinically detectable abnormalities. There has been controversy over the value and relative sensitivity of various spirometry indices. This study evaluated the forced expiratory flow between 25% and 75% of vital capacity (FEF25-75), forced expired volume in 1 second (FEV1), and the ratio between the FEV1 and the forced vital capacity (FVC) in 100 asthmatic children aged 6-17 years, 29 of whom were wheezing at the time of the evaluation. All children with clinical wheezing had a FEV25-75 < 2 standard deviations (SD) below the mean (-2 SD), whereas 8 had a normal FEV1. The majority of the wheezing children had abnormalities of all 3 indices, whether expressed as < or = -2 SD or, in the case of the FEV1/FVC, arbitrarily taken as < 80%. Sixty-seven children of the entire study group had at least 1 abnormal spirometric index, but 38 of these had no clinical abnormalities. Twelve children had a reduced FEF25-75 as the only abnormality. These results suggest that FEF25-75 is a sensitive index of airway obstruction.