Observation at necropsy of a peculiar pneumonia in infants which is believed to be a result of accidental aspiration of oily substances has been recorded infrequently. However, in the light of critical study of this condition as revealed in 7 necropsies of my own and of the data obtained on experimental animals, one is led to conclude that this is a distinctive pneumonia of relatively common occurrence, in which not only does the aspiration of oil play a dominant etiologic rôle but the clinical history and the pulmonary lesion are sufficiently constant and characteristic to merit its consideration as a clinicopathologic entity. The term "oil inspiration pneumonia" is suggested in preference to the "lipoid pneumonia" of the more recent authors.1 It may be defined as an interstitial, proliferative pneumonia, usually localized, of low grade intensity, resulting primarily from the foreign body reaction of the pulmonary tissue to the presence