The reproducibility and acceptability of airways resistance measurements using the interrupter technique (MicroR_int) obtained using a mouthpiece were compared with those using a face mask.Fifty children aged 4–7 yrs performed four sets of sixR_intmeasurements; two using a mouthpiece and two using a face mask with integral mouthpiece.Complete data were obtained from 45 (90%) children using the mouthpiece and 43 (86%) children using the mask. The two methods were equally repeatable with comparable intraclass correlation coefficients (ICC) and coefficients of variation. MeanR_intvalues obtained using the mouthpiece were significantly lower than those using the face mask ((mean±sd) mouthpiece=0.81±0.18 kPa·L⁻¹·s, mask=0.88±0.24 kPa·L⁻¹·s p=0.0002). Although the mean paired differences between the two methods were small (0.07 kPa·L⁻¹·s), the ICC and limits of agreement confirmed that the two methods could not be used interchangeably. Sixty-seven per cent of children preferred the face mask but this was more time-consuming (p=0.03). Children did not produce more repeatable results using their preferred method, nor did they improve with practice.Repeatable airway resistance measurements using the interrupter technique can be obtained from young children using either a mouthpiece or a face mask, but there are significant clinical and statistical differences between the results obtained.