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 reported 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 [8]. We have demonstrated that FENO has a significant time-of-the-day difference with a median of 12 ppb between peak and trough readings within a 24-h cycle in stable mild/moderate asthma [10]; 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.