The aim of this study was to compare two different methods, tidal breathing (TB) and single-breath (SB), for measuring fractional exhaled nitric oxide (FENO) in infants.FENO was measured in 71 infants with either recurrent wheeze (n=32), recurrent cough (n=16) or no symptoms (healthy, n=23) using both methods. For TB measurements five breaths were collected into a gas sampling bag (off-line reservoir sampling). The SB method was modified from the raised volume rapid thoraco-abdominal technique. Agreement between the two methods was investigated and both methods were used to compare FENO in infants with and without symptoms.Flow dependence of SB FENO was demonstrated using two expiratory flows (I I and 40 mL(.)s(-1)). There was a moderate correlation (r=0.60) but poor agreement between levels using the TB and SB methods. A significant difference in FENO between healthy children and children with wheeze was found using the SB but not the TB method.Due to lower expiratory flow and reduced nasal nitric oxide contamination the single-breath technique may be more sensitive than the tidal breathing method for detecting differences in exhaled nitric oxide between infants with and without respiratory symptoms.