Chronic infection of the lungs with Pseudomonas aeruginosa complicates many long-term lung diseases including cystic fibrosis, bronchiectasis, chronic obstructive lung disease, and mechanical ventilation. In acute inflammatory lung diseases, increased nitric oxide synthase (NOS-2) expression leads to excess nitric oxide (NO) production, resulting in the production of reactive nitrogen intermediates, which contribute to tissue damage. In contrast, the contribution of NO to pulmonary damage in chronic Pseudomonas infection of the lung has not been directly examined and is unclear. Although NOS-2 expression is increased in this condition, NO production is not abnormally elevated. It was hypothesized that chronic infection of the airways does not cause increased NO production but, in contrast, leads to inappropriately low NO concentrations that are pro-inflammatory. A rodent model of chronic airway infection was used to examine the effects on lung damage of augmenting or inhibiting NO production after airway infection with P. aeruginosa was well established. Three days post-infection, L-arginine, which augments NO production, or L-NAME, an inhibitor of NO production, was administered in drinking water. Lung damage was assessed 12 days later. L-arginine treatment reduced tissue damage, inhibited neutrophil recruitment, and reduced the pro-inflammatory cytokine interleukin (IL)-1 beta. Treatment with L-NAME caused loss of alveolar walls, greater vascular damage, and increased levels of the proinflammatory cytokine IL-6. Thus, in chronic airway infection, inhibition of NO production worsened lung damage, whereas augmenting NO ameliorated this damage. This is the first demonstration that augmenting endogenous NO production in chronic infective lung disease caused by P. aeruginosa is anti-inflammatory. Given that infection with this organism complicates many chronic lung diseases, most notoriously cystic fibrosis, these findings have important clinical implications. Copyright (c) 2006 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.