Patients will receive 5-ppm study gas (nitric oxide for inhalation or placebo) until Day 28 or extubation, whichever comes first. The following will be performed / recorded at specified times during the study: arterial blood gases, ventilator settings, methemoglobin, PRISM III score, and patient positioning. Selected centers will also be performing plasma cytokine assays, broncho-alveolar lavage fluid assays and a 6 month follow-up assessment.