BACKGROUND Diminished nitric oxide (NO) levels have been reported in adults with obstructive sleep apnea but no data are available for children with obstructive sleep-disordered breathing (SDB). OBJECTIVE To assess levels of serum NO metabolites in children with SDB and to explore the effects of NO metabolites, SDB and their interaction on blood pressure. METHODS Morning nitrite, the sum of nitrite and nitrate (NO(x)), and the average of evening and morning blood pressure were assessed in children with SDB referred for polysomnography and in controls without SDB. RESULTS Forty-three children with SDB (age: 5.8+/-2.1 years) had moderate-to-severe nocturnal hypoxemia (SpO(2) nadir: 85.6+/-4%), 54 subjects (6.6+/-2.7 years) had mild hypoxemia (SpO(2) nadir: 91.4+/-1.3%) and 20 subjects were controls free of SDB (6.7+/-3.7 years). Subjects with moderate-to-severe hypoxemia had significantly lower ln-transformed NO metabolites (1.4+/-0.7, nitrites; 2.6+/-0.5, NO(x)) compared to those with mild hypoxemia (1.9+/-0.8, nitrites; 3+/-0.6, NO(x)) and controls (2.2+/-0.7, nitrite; 3+/-0.6, NO(x); p<0.05). The effects of NO metabolites and SDB or their interaction on blood pressure were not significant (p>0.05). CONCLUSIONS Moderate-to-severe hypoxemia accompanying SDB is associated with reduced concentrations of morning serum NO metabolites, but NO levels do not seem to affect blood pressure.