Inhaled NO has been used in the treatment of neonates with severe respiratory failure but the response rate is variable. To determine the associations between initial response to NO (increase of 10 torr in PaO2 or 10% in SaO2, J Pediatr, 1994;124:302), diagnosis, and outcome (ECMO or death), an analysis of the current published trials was performed. Articles were identified through a Medine search and a review of abstracts presented at pediatric meetings since 1992. Eleven studies (3 in abstract form) were analyzed. The analysis was limited to infants ≥ 35 weeks of gestational age. Authors were contacted when information regarding variables studied had not been published. Diagnostic categories were reviewed and rate of response was determined independently by two of the authors. Response rate and outcome in responders and non responders with the respective 5-95% confidence interval (CI) were calculated for the overall population and for the subgroups of infants with congenital diaphragmatic hermia (CDH, n=29) and with other diagnoses (non CDH, n=112). The overall response rate was 65%. However, the studies were heterogeneous, with a 47% difference in the boundaries of the CIs between the studies with the lower and the higher rate of response (CI 0-31% and 78-100%). ECMO requirement or death was lower in responders than in non responders (24 vs 85%, p < 0.01, χ2) as well as in non CDH versus CDH infants (45 vs 77%, p < 0.01, χ2). In the CDH group, the rate of ECMO or death was not different between responders and non responders (64 vs 92%, p = 0.17). In summary, about 2/3 of the patients responded to NO administration. However, an initial response to NO was associated with a lower ECMO requirement or death only in infants with a primary diagnosis other than CDH.