Fractional exhaled nitric oxide (FeNO) is a single-breath test used in asthma diagnosis and management. Whilst a number of studies in mild to moderate asthma have demonstrated excellent repeatability of FeNO by assessing intra-device reproducibility from consecutive blows [1–3], the between-session and diurnal variability have been inconsistent [4–9]. Asthma is a highly variable disease. The excessive diurnal variability in airflow obstruction is a marker for disease severity, poor control and mortality. Diurnal variability of FeNO is also a predictor for poor asthma control . We have demonstrated that FeNO has a significant time-of-the-day difference with a median of 12 parts per billion (ppb) between peak and trough readings within a 24-hour cycle in stable mild/moderate asthma ; whether this same-day variation is clinically significant in the diagnosis and management of asthma remains unclear. FeNO has been readily used in the monitoring of severe asthma, but its same-day reproducibility as a single-breath test is unknown in this group.