Lung injury commonly occurs in the setting of systemic inflammatory response syndrome occurring during bacterial sepsis. There has been little work quantifying different leukocytes within the different compartments of the lung and their association with overt lung injury in sepsis. We examined the pathogenesis of lung injury after cecal ligation and puncture (CLP), a clinically relevant model of sepsis. To assess the sequestration and migration of leukocytes, leukocyte differentials were obtained for the lung vascular compartment and the bronchoalveolar airspace. At 24 h post CLP, there were signs of edema in the lung, while at 48 h after CLP, there were clear indications of alveolar wall thickening with increased cellularity and diffuse alveolar hemorrhage. The number of lymphocytes in the pulmonary vascular compartment dropped by 50% and doubled in the (bronchoalveolar lavage) BAL, 24 h after CLP compared to sham controls suggesting that there was transendothelial migration of lymphocytes. At 48 h after CLP, lymphocyte numbers in the vasculature was similar to controls but BAL lymphocyte numbers were still raised. The number of pulmonary intravascular neutrophils were similar to controls at 24 h post CLP but were greatly elevated 48 h after CLP. The increase in neutrophils was partly due to a substantial increase in the percentage of immature band cells, indicating recruitment of neutrophils from the bone marrow. There were very few neutrophils in the BAL of sham controls and CLP rats. Perfusate monocyte/macrophages were significantly increased 48 h after CLP and a similar increase in macrophages was observed in the BAL. These results strongly suggest a role for lymphocytes and macrophages in the development of overt lung injury as the migration of these cells corresponds to that of the appearance of lung injury 48 h after CLP. Importantly our data also demonstrates the compartmentalization and migration of different inflammatory cell-types during the development of sepsis.