Background: North American standardised testing protocols assume that only nasal Nitric Oxide (nNO) measured during expiration against a mouth resistance (ExRe) in children older than 5 years is a reliable tool for diagnosis of Primary Ciliairy Dyskinesia (PCD). Objectives: To establish the relevance of tidal breathing (TB) nNO measurement as a first line test in young children. Methods: Monocenter retrospective study (2013 - 2019) based on the records of children seen for suspicion of PCD. nNOExRe and nNOTB were measured using CLD 88 sp (EcoPhysics, Suisse). Results: Nasal NO measurement was possible on 460 occasions in 382 children (median [IQR] (range) age of 7.1 [4.8 to 11.1](0.1 to 19.4) years, 167 (43.7%) females). Recorded numbers of nNOExRe and nNOTB were 247 (53%) and 355 (77%); respectively, with 158 (34%) nNOTB on isolation. One hundred and five children 5 years or less performed 24 (19%) nNOExRe and 119 (94%) nNOTB. Among the 81 children with only nNOTB result and based on all available investigations, PCD diagnosis was excluded (n=58, nNOTB 298 [199;391]ppb), confirmed or very probable but pending (n=12 and n=3, nNOTB 42 [28;69]ppb), or unknown (n=8, nNOTB 129 [31;239]ppb). Among children with excluded versus confirmed / pending PCD diagnosis, frequencies of ciliary beat study, electron microscopy, and genetic testing were 15 (26%) versus 9 (60%), 3 (20%) versus 8 (53%), and 0 versus 8 (53%), respectively. Conclusion: In children less than 5 years, the success rate of nNOTB measurements was 94% and allowed stopping PCD workup in 74% of cases in children with medium-low clinical suspicion.