The possibility to exclude PCD by a simple measurement of nasal NO concentrations seems clinically very attractive, because the diagnosis of PCD is a difficult one and involves the complex assessment of ciliary structure and function. Electron microscopy is an essential part of diagnostic testing to ensure that secondary ciliary dysfunction from acute or chronic infection is not confused with PCD. Ultrastructural studies have to demonstrate a clear-cut abnormality of ciliary structure, but in rare cases this is not always possible. In addition, primary ciliary dyskinesia has been described without ultrastructural abnormalities, sometimes with primary ciliary disorientation.15–16 There is only one other test that has been used to screen patients for PCD, the saccharin test. This test is almost impossible to perform properly in children because of its duration and need for subjective interpretation. Nasal NO levels > 105 ppb excluded PCD with a 100% certainty in our study population, and would have avoided more invasive and costly investigations for these patients. The availability of an easy and noninvasive screening test for PCD is also attractive, because this may facilitate early diagnosis. The measurement of nasal NO may also be valuable in patients with inconclusive biopsy results. It must, however, be considered that low levels of nasal NO may also occur in children with cystic fibrosis, non-PCD bronchiectasis, or sinus disease of any etiology due to obstruction of sinus ostia reducing transfer of NO from the paranasal sinuses. We found low levels of nasal NO in two children without PCD, one with plastic bronchitis due to severe asthma, and the other believed to have chronic sinusitis and nonallergic asthma with incomplete response to bronchodilators. Although the reason for the low nasal NO concentrations is unknown, it is recognized that nasal NO correlates with impaired mucociliary function, and that reduced NO synthesis may contribute to the chronic airway infections present in both our children. Nakano et al13 described low levels of nasal NO in patients with panbronchiolitis and hypothesized that whatever the pathogenesis of a disease, chronic sinusitis per se can potentially decrease nasal NO concentrations because of the widespread epithelial damage in paranasal sinuses resulting from recurrent inflammation.,13 In fairness, we are unable to exclude that these two children are affected by a rare form of PCD characterized by abnormal ciliary orientation and absence of ultrastructural defects.