Cardiovascular diseases are frequent in patients with obstructive sleep apnea syndrome (OSAS), but the mechanisms underlying this association are largely unknown. Nitric oxide (NO) is a key regulatory element of vascular physiology. The concentration of NO in the exhaled air ([NOexh]) appears to be reduced in patients with systemic and pulmonary hypertension. This study sought to investigate whether [NOexh] is abnormal in patients with OSAS, and to explore potential relationships between [NOexh] and the severity of OSAS. We measured [NOexh] in 24 patients with OSAS (apnea-hypopnea index (AHI), 55 +/- 4 hour-1) (x +/- SEM), and in 7 healthy volunteers in whom OSAS was excluded clinically. [NOexh] was measured on line by a chemiluminescence analyzer (Dasibi Environmental Corporation, Glendale, Calif). Seven patients with OSAS (29%) had a positive history of cardiovascular disease. Mean [NOexh] was 19.7 +/- 3.2 ppb in healthy subjects, and 22.2 +/- 3.0 ppb in patients with OSAS (p = ns). [Noexh] was not significantly different in those patients with or without cardiovascular disease. [NOexh] was not significantly related to the AHI, the body mass index, or the arterial O2 saturation at night. These results show that [NOexh] is not abnormal in patients with OSAS, and that it does not relate to the presence of cardiovascular disease or to any of various common indices of disease severity.