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Background and objective : Cross-sectional and longer-term studies have demonstrated abnormal yet stable multiple-breath inert gas washout (MBW) indices in patients with primary ciliary dyskinesia (PCD). This study aimed to assess the intermediate term evolution and the between-occasion variability of MBW indices in PCD over 1 year. Methods : Children and young adults with a confirmed diagnosis of PCD were included in this single-centre, prospective, observational, longitudinal study. Over 1 year, nitrogen (N 2 ) MBW and spirometry were performed at three occasions during ordinary scheduled outpatient visits. Trends and variability in lung clearance index (LCI), moment ratios, normalized N 2 concentration at six lung volume turnovers, and regional ventilation inhomogeneity indices of the conducting and intra-acinar airways (S cond *V T and S acin *V T ) were analysed using linear mixed models. Results : Forty-two patients, aged 6-29 years (median: 15.4), performed 116 N 2 MBW test occasions and 96.6% were technically acceptable. A minimal, although significant, increase in LCI over 1 year (mean: 0.51 units, 95% CI: 0.12-0.91, p  = 0.01) was found; while, all other N 2 MBW indices and FEV 1 remained unchanged. A moderate correlation was observed between LCI and FEV 1 ( r  = -0.47, p  = 0.0001). The limits of agreement between tests 1 year apart were for LCI: -1.96 to 2.98; S cond *V T : ± 0.039; S acin *V T : -0.108 to 0.128. Conclusions : Children and young adults with PCD managed at a specialist centre showed slightly, but significant, increasing LCI and otherwise unchanged ventilation inhomogeneity indices and dynamic volumes over the intermediate term of 1 year. Estimates of the variability of N 2 MBW indices may inform sample size calculations of future randomized controlled trials.