Methodologies used to assess airway responsiveness (AR) in children administer the same dosage schedule to all children despite the great range in the size of subjects. The aim of this study was to examine the hypothesis that the level of AR is size dependent within same-age cohorts and between different ages. Among a birth cohort of 1,037 New Zealand children participating in a longitudinal study, 818 had at least two measurements of airway responsiveness between ages 9 and 15 yr. Each child performed spirometry and a four-dose methacholine inhalation test. A continuous slope index of methacholine responsiveness was computed. AR slope indexes were analyzed using longitudinal methods, which included an indicator variable for subjects who reported having any wheeze. AR was lowest in both males and females in the upper quartile for height than those in the lower quartile, independently of age. AR tended to be higher (responded to lower concentrations of methacholine) in boys than girls and to decline with age among wheezers. The greater level of responsiveness in smaller or younger children could be explained by these individuals having received a dose of methacholine that was relatively large for their size.