Polymorphonuclear leukocytes (PMN) enter the circulation from the bone marrow and remain within the vascular space unless they become activated and migrate out of the vessels as part of an inflammatory response. This article reviews their behavior in the pulmonary circulation of both healthy and diseased individuals. First, the flow of PMN is compared with that of erythrocytes (RBC) to explain how differences in the traffic of these two cell types result in a concentration of PMN with respect to RBC in pulmonary capillary blood. Second, some recent concepts are presented concerning the role of PMN in the pathogenesis of emphysema, and the hypothesis is discussed that the inhalation of cigarette smoke contributes to the pathogenesis of emphysematous lung destruction by delaying and activating PMN while they travel through lung capillaries. Finally, the example of streptococcal (i.e., pneumococcal) pneumonia is used to illustrate new information as to how PMN adhere to the endothelium and migrate into an inflammatory site in the lung. The effect of this localized inflammatory response on PMN traffic in the surrounding lung also is discussed in relation to the pathogenesis of the generalized lung injury (acute respiratory disease syndrome [ARDS]) that can complicate severe pneumococcal lung infections.