In a series of consecutive autopsies of 87 adults, 43 showed appreciable numbers of alveolar macrophages which were unrelated to active inflammation. In most instances atelectasis was the underlying cause of accumulation of macrophages. Other causes were alveolar edema; thickened alveolar walls; proximity to bronchi, vessels or scars; and no demonstrable cause. The common factor in all instances in which demonstrable causes were recognized was reduction in the excursions of alveolar tissue. A theory is descr. which satisfactorily explains the normal mechanism for dust and cell removal from the alveolus to the bronchiole and takes into account the demonstrated relationship between alveolar stasis and reduced excursionary activity of alveolar tissue.