Although most wheezy infants are considered asthmatic, they generally respond poorly to antiasthma treatment, and there is inadequate knowledge about the pathologic mechanisms that cause wheezing at this age. The aim of this study was to determine whether the strong association between wheezing and bronchial responsiveness (BR) seen in older subjects was also present in infants. We compared BR with inhaled histamine in 19 recurrently wheezy infants with a group of age-, height-, weight-, and sex-matched control infants. Maximal flow at FRC (VmaxFRC) was determined from partial expiratory flow-volume curves generated using the "squeeze" technique. Histamine was delivered during 1 min of tidal breathing in doubling concentrations from 0.125 g/L to a maximum of 8 g/L or until VmaxFRC fell by 40% (PC40). The median baseline VmaxFRC for the wheezy infants was 100.0 ml/s compared with 182.0 ml/s for the control infants (p less than 0.01). However, there was no significant difference in the PC40 between the two groups (2.1 versus 2.3 g/L).