Although interlobar fluid accumulation has a varied etiology, it is most frequently encountered in patients with heart disease. In this condition it may be the end-result of an inflammatory process or, more commonly, it may be the transudative type found in decompensated individuals.

Interlobar effusions arising by transudation differ in no way from those in the free pleural cavity except in location. Some are walled off in the interlobar region by an adhesive pleurisy of the general pleural space; others merely enter the fissure as an extension from a hydrothorax.

In the course of heart failure pleural reactions are very frequent, not only in the interlobar region but also in the free pleural space. Upon resorption, only scars or thickened pleura remain. Whenever such residua are found, it is reasonable to assume that the patient had either preexisting fluid (interlobar or otherwise) or chronic pulmonary stasis.

Interlobar effusions and their pleural thickenings are not roentgenological freaks, but constitute an integral part in the history of chronic cardiac disease.