You will find few chest radiographic reports that do not include the term atelectasis. Atelectasis means incomplete lung expansion. This term is frequently tossed about, and little thought is given to its significance. It is important to think about the underlying pathophysiology any time the term atelectasis is employed in describing a chest radiograph finding. Pneumonia and atelectasis are always considered in the differential diagnosis of airspace opacities. It can be difficult to differentiate between the two processes without examining the patient for signs of pneumonia (fever, elevated WBC, lung auscultation findings, etc.).