We attempted to measure the distribution of ventilation in 24 children who had cystic fibrosis (CF) and 58 control subjects (ages 3.9-6.8 years) using the multiple breath nitrogen washout (MBNW) test. Moment analysis was performed on the MBNW curve, and the first and second moment ratios were derived. Only 40 (49%) of the children were able to complete the test satisfactorily and success was related to age (P less than 0.01). Children who had CF had higher moment ratios, both first and second, than did controls (P less than 0.001); also, CF subjects who had more severe symptoms had higher moment ratios than did those who had minimal symptoms (P less than 0.005). The first moment ratio had a lower coefficient of variation than did the second (14% vs 31%) and, in 8 children who performed the test in duplicate, it also had a lower index of intrasubject variability (6.4% vs 15.1%). Both ratios were lower than those described in older children and in adults, which is consistent with the more rapid washout seen in this age group. Although moment analysis of the MBNW curve can be performed in approximately 50% of children 4 to 6-years-old, the complexity of the test, the apparatus, and the analysis and the high failure rate in children under the age of 6 are likely to restrict its use to research studies in specialized pediatric respiratory laboratories.