Airways resistance measured by the interrupter technique (Rint) requires little patient cooperation and has been successfully used in young children, but little studied in infants. The authors aimed to evaluate the measurement of Rint in infants, using a commercially available device (the MicroRint), by comparing it with an established technique to measure respiratory resistance: the single breath occlusion technique (SBT); and a measure of airflow obstruction during forced expiration. Infants <18 months old with a history of wheeze, sedated with triclofos for pulmonary function testing, had measurements taken and compared to Rint (using the MicroRint), respiratory system resistance (Rrs) by SBT, and to maximal flow at functional residual capacity (V'maxFRC). Paired data from 25 of 37 infants studied was obtained. There was a significant difference between Rint (mean 2.94+/-0.68) and Rrs (4.02+/-0.87), but the two measures were strongly correlated (r=0.7). Rint was negatively correlated with V'maxFRC (r=-0.63). Smaller infants failed to trigger the MicroRint. Interrupter resistance values in infants are significantly lower than values of respiratory system resistance obtained by passive mechanics. However, there is a strong correlation between the two measurements, as well as between resistance measured using the interrupter technique and maximal flow at functional residual capacity, which indicates that resistance measured using the interrupter technique may be a useful marker of airway obstruction in infants. There remain a number of theoretical and technical problems which require further exploration.