Respiratory inductive plethysmography (RIP) is a simple technique for an objective, noninvasive assessment of thoracoabdominal asynchrony, which in turn is an indirect measure of airway obstruction. We evaluated different indices of asynchrony obtained by RIP before and after methacholine-induced airway obstruction. Bronchial obstruction was elicited by progressive doubling concentrations of methacholine until a > 15% fall in the transcutaneous oxygen tension (PtcO2) had developed. Maximal expiratory flow rates at functional residual capacity (FRC) (VmaxFRC) was obtained by the squeeze technique before and after the challenge. Fifteen infants with a history of wheezing were studied after sedation. Thoracoabdominal movements were recorded with RIP bands placed around either the upper or the lower ribcage (RC) and around the abdomen (AB). An inspiratory asynchrony index (IAI) and an expiratory asynchrony index (EAI) were calculated as determined by the lag of RC relative to AB at start of inspiration and of expiration, respectively. The total time in asynchrony (TTA: the percentage of time in which the RC and the AB signals were in opposite direction) and phi (an angle derived from a Lissajous loop) were also calculated. All subjects responded to the challenge. The median fall in PtcO2 following methacholine challenge was 23.6% and in VmaxFRC was 43%. A large scatter of baseline values was found for all indices with the exception of TTA. There was no correlation between TTA and age, length, or VmaxFRC. The IAI and EAI with the RC band in the upper position were the most sensitive indices, both within subjects (65% of the subjects had a significant change in IAI and 80% in EAI) and for the group as a whole (median values increased for IAI, P = 0.007, and for EAI, P = 0.017). TTA and phi were less sensitive, and a great discrepancy was observed between the two measurements. Poor results were obtained with the RC band in the lower position. No correlations were found between the changes in IAI and EAI, with the RC band around the lower chest and VmaxFRC. We conclude that IAI and EAI, measured with the RC band in the upper position and another band around the abdomen, can detect changes in thoracoabdominal asynchrony in most infants. The usefulness of assessing IAI and EAI in infants with acute lower airway obstruction needs to be determined.