Considerable evidence supports the presence of oxidative stress in cystic fibrosis (CF). The disease has long been associated with both increased production of reactive oxygen species and impaired antioxidant status, in particular during the chronic pulmonary infection with Pseudomonas aeruginosa, which is the main cause of morbidity and mortality in CF. Guinea pigs are unable to synthesize ascorbate (ASC) or vitamin C, a major antioxidant of the lung, and thus like human beings rely on its presence in the diet. On this basis, guinea pigs receiving ASC-deficient diet have been used as a model of oxidative stress. The aim of our study was to investigate the consequences of a 7-day biofilm-grown P. aeruginosa lung infection in 3-month-old guinea pigs receiving either ASC-sufficient or ASC-deficient diet for at least 2 months. The animals receiving ASC-deficient diet showed significantly higher mortality during infection and increased respiratory burst of peripheral polymorphonuclear neutrophils (PMNs) compared with the animals receiving ASC sufficient diet. The inflammatory response at the site of lung infection consisted of PMNs and mononuclear leucocytes (MN), and higher PMN/MN ratios were present in animals on ASC-deficient diet compared with animals on ASC sufficient diet. Measurements of the ASC levels in the lung were significantly decreased in infected compared with non-infected animals. Interestingly, the infection by itself decreased the antioxidant capacity of the plasma (measured as plasma oxidizability) more than the ASC-deficient diet, suggesting a high consumption of the antioxidants during infection. Our data show that poor antioxidant status exacerbates the outcome of biofilm-related infections.