Recently there has been an intense research what is the role of nitric oxide (NO) in physiological and pathological processes is, especially in pneumonology. Authors discussed endogenous NO synthesis, its molecular mechanism of action, as well as the importance of its detection in exhaled air and clinical utility of such measurements in asthma (AO) and chronic obstructive pulmonary disease (COPD). There is evidence that eNO is increased in asthma, correlates with other inflammatory markers and corticosteroids, and anti-leukotrienes reduce its levels. In contrast, assessments of eNO in patients with COPD revealed conflicting results. The authors pointed out, that the eNO level analysis is easy to perform, may be used to diagnose and monitor disease activity and the effects of anti-inflammatory therapy. However this method has limitations resulting from high costs.