Asthmatics have increased gaseous nitric oxide in their exhaled breath. Measurements of the fraction of nitric oxide in exhaled breath (FE(NO)) are used for the diagnosis and management of asthma. One analyser (the NIOX chemiluminescence analyser) has a feature (restriction of instant flow, IF) which we suspected was responsible for reducing the success rate for obtaining FE(NO) measurements whilst not affecting the measurement itself. In a study involving 38 children, median age 10 years, we observed that FE(NO) values taken in the same individual with and without IF restriction did not differ. We also observed that the success in obtaining a FE(NO) measurement was higher (57%) when IF was not restricted and was lower (41%) when IF was restricted, p=0.002. Our findings do not support the use of restriction of IF on the NIOX analyser.