Pulmonary intravascular macrophages reside in the pulmonary capillaries and phagocytize bacteria and particulates. These cells are prominent in several animals species, but they have not been described in humans. Samples of lung tissue from patients undergoing thoracotomies for excision of noninfectious diseases were examined with transmission electron microscopy to determine if pulmonary intravascular macrophages are present in humans. The macrophages, with cytoplasm closely adjacent to the capillary and with an irregular contour, were seen in specimens from all patients. The morphologic features of human pulmonary intravascular macrophages resemble the appearance of these cells in animals. The potential significance of pulmonary intravascular macrophages is discussed with regard to sepsis-induced acute respiratory failure.