When in transit in the pulmonary capillaries, neutrophils can interact with the endothelium and gas in the alveoli. Neutrophils play a major role in lung defences and have been implicated in the pathogenesis of lung injury, such as emphysema and the adult respiratory distress syndrome. Moreover, the pulmonary capillaries receive all of the circulating neutrophils. It is surprising therefore, that relatively little is known of the factors which influence the passage of neutrophils through the pulmonary circulation. This article reviews our knowledge of neutrophil kinetics in the lungs and describes techniques using radiolabelled cells to quantify neutrophil traffic in the pulmonary circulation which should help to elucidate the pathogenesis of conditions where neutrophils are thought to play a role in lung injury.