CRD summary: The authors concluded that nitric oxide is associated with limited improvements in oxygenation in patients with acute lung injury and acute respiratory distress syndrome, but there is no improvement in mortality and it may cause adverse effects. This was a well-conducted and clearly reported review and the authors' conclusions are likely to be reliable. CRD commentary: The review addressed a clear question that was defined in terms of the participants, intervention, outcomes and study design. The authors searched several relevant sources for published and unpublished studies and made attempts to minimise language bias. They used methods to minimise reviewer errors and bias in the study selection, validity assessment and data extraction processes. Validity was assessed using specified criteria and the results of this assessment reported. The studies were, in general, appropriately combined in a meta-analysis and statistical heterogeneity was assessed. Significant heterogeneity was found for duration of ventilation but potential reasons for this were not discussed. This was generally a well-conducted and clearly reported review and the authors' conclusions are likely to be reliable.