Exhaled nitric oxide (FeNO) is a validated marker of eosinophilic inflammation. Fenom Pro is a novel FDA-cleared monitor for FeNO. The American Thoracic Guidelines from 2005 recommend at least 6 s exhalation for adults and in some cases up to 10 s, and 4 s for children, and that the average of the first two valid exhalations is taken as the FeNO value.Clinical precision, 6 versus 10 s exhalations, the first versus the average of the first two valid exhalation methods comparison were evaluated for Fenom Pro, as well as a methods comparison to the NIOX VERO monitor.The intent-to-treat population (n = 126) consisted of 83 adults, and 43 pediatric subjects with 16 subjects under 12 years of age. Clinical precision for 10 s exhalations on Fenom Pro was excellent with a within-subject standard deviation (SD) range of 0.57-3.73 ppb and mean coefficient of variation (CV) range of 4.21% to 9.65%. The clinical precision for the separate adult and pediatric groups as well as for the 6 s exhalations were similar. The 10 and 6 s exhalation comparisons and one versus the average of two valid exhalations showed a high level of agreement. The Fenom Pro and the NIOX VERO monitors also demonstrated a high level of agreement with the values from the latter slightly lower (mean bias of -3.2 ppb).Fenom Pro demonstrated eminently acceptable performance supporting its clinical utility. The data suggests that 6 s exhalations can be used in adults and children, and that one exhalation is adequate rather than obtaining the average of two exhalations on Fenom Pro.