The administration of oxygen to infants via nasal cannulas is now a common practice in neonatal units although the inspired oxygen concentration reaching the patient's airway is unknown. We measured the hypopharyngeal oxygen concentration in 10 infants who were receiving oxygen via nasal cannulas and assessed the impact of changes in the flow rate and inspired oxygen concentration. Weaning these infants by reducing the flow rate, even if changes are slight, produces clinically important changes in the oxygen concentration reaching the airway. Such changes are poorly tolerated by infants with chronic lung disease. Changing the flow rate and inspired oxygen concentration, rather than the flow rate alone, provides greater precision and is likely to avoid excessive and abrupt changes in the oxygen concentration reaching the airway.