Background The number of immunosupressed patients has increased in the past decades. Among them Pseudomonas aeruginosa (P. aeruginosa) is one of the leading bacteria for pneumonia that are associated with poor prognosis. However, the pathogenesis of P. aeruginosa pneumonia in immunosupressed patients is not understood completely. Previous reports showed keratinocyte growth factor (KGF) is associated with lung injury in immunocompetent hosts. In this study, we investigated the different reactions of lung injury, lung pathology and KGF expressions in P. aeruginosa pneumonia between immunosuppressed and immunocompetent rats.Methods Immunosuppression of male rats was induced by injecting immunosuppressive subcutaneously.Pneumonia was established by instilling P. aeruginous tracheally. The immunocompetent rats were the control group. Survival rate, lung histopathology, pulmonary permeability and oedema, KGF mRNA and protein expressions in lungs of both groups were investigated.Results The survival rate of immunosuppressed group was lower than that of immunocompetent group (33.3%vs 83.3%). After exposure to bacteria, pulmonary permeability and wet/dry ratio in immunosuppressed group were higher than those in immunocompetent group. Pulmonary congestion and haemorrhage were more intensive in immunosuppressed group compared to immunocompetent group. Apoptosis and necrosis were also observed in infected lungs of immunosuppressed rats. Although we detected KGF expressions in lungs of both groups after infection, the expressions of KGF protein and mRNA gene in immunosuppressed group were much lower than in immunocompetent group.Conclusions Compared with immunocompetent group, there was more intensive lung injury in immunosuppressed group. Severe lung injury may contribute to the poor prognosis of pneumonia. KGF expressions of pneumonia in immunosuppressed rats were less than those in immunocompetent ones.