While it is known that exhaled nitric oxide (ENO) is increased in adults and school children with asthma exacerbation probably as an expression of disease activity, no studies have investigated whether this phenomenon also occurs in infants and young children with recurrent wheeze exacerbation. We measured ENO in 13 young children (mean age 20.2 mo) with recurrent wheeze (Group 1) during an acute episode and after 5 d of oral prednisone therapy. ENO was measured also in nine healthy control subjects (Group 2) (mean age 16.9 mo) and in six children with a first-time viral wheezy episode (Group 3) (mean age 11 mo). To measure ENO, infants inhaled NO-free air via a face mask from a reservoir and, through a nonrebreathing valve, exhaled in a collecting bag that was analyzed by chemiluminescence. To address the question of whether the levels of ENO collected in the bag are a reflection of the pulmonary airway, ENO determinations were performed in two healthy infants before and after tracheal intubation for elective surgery. During the acute episode of wheezing the mean (+/- SEM) value of ENO in children with recurrent wheeze (Group 1) was 14.1 +/- 1.8 ppb, almost threefold higher than in healthy control subjects (5.6 +/- 0.5 ppb, p < 0.001). After steroid therapy we found a mean fall of 52% in ENO (5.9 +/- 0.7 ppb, p < 0.01) compared with baseline values. ENO values measured before and after intubation in two infants were 6 ppb and 5 ppb in one child and 7 ppb and 6 ppb in the other one. The mean value of ENO of children with first-time wheeze (Group 3) was 8.3 +/- 1.3 ppb, significantly lower (p < 0.05) than the value of children with recurrent wheeze (Group 1). In conclusion, we describe a method to measure ENO in young children and show that infants with recurrent wheeze have elevated levels of ENO during exacerbation that rapidly decrease after steroid therapy. This suggests that, in these children, airway inflammation could be present at a very early stage.