Exhaled nitric oxide (eNO) is elevated in patients with inflammatory pulmonary diseases and it has attracted increasing interest as a simple, noninvasive marker of airway inflammation. Little is known, however, about factors that might affect eNO in healthy subjects. We measured eNO in 157 healthy 7- to 13-yr-old children (mean 9.7 yr, 77 girls), with no history of respiratory tract disease, using a recently validated, single-breath technique. Measurements of eNO were obtained at driving (mouth) pressures of 10, 15, and 20 cm H2O and 3 eNO plateaux were achieved for each child at each pressure. Exhaled NO decreased with increasing pressure (increasing expiratory flow) (p < 0.001) and increased with age (p < 0.001). Concentrations were greater in children with a positive skin prick test (p < 0.0001). Geometric mean eNO levels were 7.2 ppb in children with no positive skin prick tests (n = 116), 10.9 ppb in children with one positive reaction (n = 24), and 20.1 ppb in children with two or more skin reactions (n = 17). Age and immunological reactions to common allergens are associated with increased eNO in children and should be controlled for in studies of eNO. The mechanisms responsible for these associations require further study.