Postmortem examination was performed in 15 patients who had plate atelectasis (PA) on their last antemortem radiograph. In 10 patients, corresponding abnormalities were found in the lungs. In all 10, there was peripheral subpleural linear collapse combined with invagination of the overlying pleura; prominent interlobular septa were observed within or bordering the linear atelectasis in nine. The atelectasis and the invaginated pleura both contributed to the linear radiographic density. The bronchi supplying the areas of PA showed no obstruction nor rearrangement. Despite a frequent association with pulmonary embolism (6/10), there was no evidence that PA directly represented thrombosed vessels or infarcts. The findings suggest that PA represents one (radiographically visible) form of peripheral atelectasis. In persisting low-volume states, the dependent subpleural lung appears to buckle and fold in. The frequent association of PA with congenital pleural clefts, indentations, scars, and incomplete fissures suggests that this process may preferentially occur at sites of pre-existing pleural invagination. PA frequently indicates more widespread peripheral atelectasis than is radiologically apparent; it also may indicate the presence of some other (more serious) abnormality in the chest or abdomen.