Congestive heart failure is the most common cause of an interlobar pleural effusion. The correct diagnosis is usually evident on the lateral chest film which shows the characteristic spindle-shaped shadow within the plane of the fissure and continuous with a line of thickened pleura at one or both ends. In the frontal view, however, the identity of such an effusion, especially when it involves the long fissure, is considerably less evident. The possibility of an interlobar effusion should be considered whenever a poorly defined shadow is seen in the central portion of the lung and is associated with evidence of pulmonary congestion and cardiomegaly.