Eight patients had air-fluid levels in the lung as a result of fluid accumulation in preexisting bullae. In four cases the cause was peribullous pneumonitis; in the other four cases the cause could not be determined. All patients had a favorable clinical course while intrabullous fluid was present. The initial interpretation of air-fluid levels was correct in only two instances. Differentiation of fluid-containing bullae from other causes of air-fluid levels is important so that unnecessary diagnostic and therapeutic maneuvers can be avoided.