Breathing pattern variability was determined in 10 asthmatic adolescents during repeated bronchial histamine and methacholine challenges (HiCh/MeCh). The purpose was to provide information on ventilatory control in asthmatics by comparing the variability of the various breathing pattern parameters at rest and during induced bronchial obstruction. Changes in variability during bronchial obstruction might be explained by either anxiety effects causing increased variability or by the minimization of the work of breathing causing decreased variability. Ventilation was monitored by respiratory inductive plethysmography in order to minimize the effects on the spontaneous pattern of breathing. Breath-to-breath and day-to-day variability were determined concerning respiratory frequency (fR), inspiratory tidal volume (VTI), inspiratory ventilation (V'I), inspiratory time to total cycle time ratio (TI/TTOT), mean inspiratory flow (VTI/TI, an index of ventilatory drive), rib cage fraction of VTI (VRC/VTI), and maximum compartmental amplitude to VTI ratio (MCA/VTI; an index of rib cage and abdominal phasing). No difference in any parameter was found regarding breath-to-breath coefficient of variation (CV = SD/mean) between recordings at baseline, after saline inhalation and after threshold dose of the provocative agents, i.e. > 20% fall in FEV1. Variability was less for MCA/VTI and VRC/VTI (mean CV 1.3 and 7.7%, respectively) than for TI/TTOT, fR, VTI/TI, VTI, and V'I (14.2, 15.8, 20.9, 22.2 and 21.1%, respectively) (P < 0.01). Likewise, the day-to-day variability did not differ in any parameter between recordings at baseline, after saline inhalation and after threshold dose. The variability was less for MCA/VTI (0.7%) than for TI/TTOT, VRC/VTI, V'I, VTI/TI, fR and VTI (7.1, 12.1, 12.8, 14.2, 13.0 and 15.4%) (P < 0.05). Furthermore, TI/TTOT was less variable than VTI (P < 0.05). Thus, the ventilatory pattern was quite reproducible on a day-to-day basis, despite considerable breath-to-breath variability. Ventilatory drive and tidal volumes were more variable than the rib cage and abdominal phasing, the respiratory timing and the rib cage fraction of tidal volume. The lack of difference in variability between rest and induced bronchial obstruction indicates that other factors than anxiety or minimization of the work of breathing are important for the control of respiration in asthmatics during bronchial challenge.