The aim of this study was to determine whether oral supplementation with EPA/DHA (10.5 and 5.1% of fat, respectively) could improve the outcome of pulmonary P. aeruginosa infection in cftr(-/-) mice compared with wild-type (Wt) mice similarly treated. Because gender could influence the susceptibility of cftr-deficient mice, results were analyzed by gender. Wt and (-/-) mice were randomized for 6 wk to consume a control or EPA/DHA diet, infected with endotracheal injection of 5 × 10(7) CFU/mouse of P. aeruginosa, and killed 24 h later. Cftr(-/-) mice were more susceptible to infection than were Wt mice; (-/-) males had more neutrophils (P < 0.01) and a higher keratinocyte-derived chemokine (KC) level (P < 0.05), and (-/-) females had greater lung injury and mortality (P < 0.05). Female (-/-) mice were more susceptible than (-/-) males with a higher mortality and lung injury (P < 0.05). The EPA/DHA diet reduced neutrophil numbers and KC and IL-6 levels (P < 0.05) in (-/-) males and reduced mortality rate (P < 0.001), lung permeability, and IL-6 level (P < 0.05) in (-/-) females compared with (-/-) mice fed the control diet. These results were associated with a reduction in the pulmonary bacterial load (P < 0.05), an increase in the EPA/DHA concentration in cell membranes of (-/-) males and females (P < 0.01), and an increased weight gain only in males compared with (-/-) mice fed the control diet (P < 0.01). In conclusion, EPA/DHA improves the host resistance of (-/-) mice, although the beneficial effect differed in males and females.