Inhaled nitric oxide (I-NO) is a selective pulmonary vasodilator, effective in treatment of PFC. NO has several potential side effects that have not been fully evaluated in patients. Nitric Oxide, produced in large quantities by activated inflammatory cells, is metabolized in presence of superoxide to toxic peroxynitrite that among others nitrates tyrosine, forming nitrotyrosine. NO may also modify synthesis of cytokines. The components in the epithelial lining of the airspaces are particularly exposed to I-NO. The aim of this prospective study was to investigate whether I-NO therapy is associated with toxicity, evidenced by increase in nitrotyrosine levels or by increase in the inflammatory cytokines. Altogether 12 term infants with PFC received I-NO: initial dose 20 ppm for a mean of 12 h, followed by 6 ppm and 1 ppm for a mean of 55 h. Twelve conventionally treated controls served as controls: they were matched for the gestation, etiology of PFC, and for the severity of the disease. The airway specimens and urine were collected daily during the first week. The specimens were analyzed for interleukin-1 receptor antagonist (IL-1ra), Granulocyte Macrophage-Colony Stimulating Factor(GM-CSF), and IL-1β. The airway specimens were analyzed for proteinassociated nitrotyrosine using an immunoassay (ELISA, Western blot). There was no detectable difference in cytokine concentrations from airway specimens or from urine between the I-NO- and the conventionally treated infants. During I-NO, the mean GM-CSF in the airway specimens was 21.0 pg/ml(± 7.6 SEM) (control 30.5 ± 8.4). Nitrotyrosine was barely detectable in proteins of the airway specimens both in the I-NO treated (0.2± 0.2 arbitrary units, AU) and in the control infants (0.4 ± 0.1 AU) during the period of NO therapy. After the I-NO therapy, there were no differences in nitrotyrosine or the cytokine levels, compared to the controls. NO is an important intra- and transcellular messenger with multiple functions. NO additionally is a free radical that under particular conditions forms highly reactive toxic compounds. We propose that I-NO as used in the present study: 1) does not increase proinflammatory cytokines, IL-1 and GM-CSF; 2) does not increase protein nitration indicative of NO-induced damage.