BACKGROUND Multiple breath washout (MBW) is an attractive test to assess ventilation inhomogeneity, a marker of peripheral lung disease. Systematic research on MBW feasibility in preschool children is scanty. OBJECTIVES We assessed feasibility of nitrogen MBW in children aged 3-7 years in a clinical setting applying current ERS/ATS consensus. METHODS Sixty-two children with asthma were enrolled in the Swiss rehabilitation center Hochgebirgsklinik Davos without MBW experience and prior to consensus release. Age range was 3.1-6.7 years. All children were naïve to MBW, received training, and then aimed for triplicate nitrogen MBW within 20 min. We afterward assessed the number of MBW attempts and successful tests according to consensus asking for three technically acceptable trials with functional residual capacity (FRC) measurements within 25% of median FRC. Secondly, factors related to success rate and FRC variability were assessed. RESULTS Out of 205 MBW attempts in 62 children, 103 trials (50%) in 51 children were successful. Only 15 children (24%) achieved three valid trials as suggested by consensus. At least two valid trials were obtained in 37 children (60%). Age was positively correlated with success rate. FRC variability was inversely correlated with variability of tidal volume. CONCLUSIONS MBW was only feasible in one quarter of inexperienced children