Fourteen patients in whom air-fluid levels developed in pre-existing emphysematous bullae were prospectively identified and followed. Although two patients were asymptomatic, 12 patients presented with one or more symptoms of fever, dyspnea, cough, pleuritic chest pain, and/or purulent sputum. Three patients had focal pulmonary abnormalities on physical examination; three patients had elevated leukocyte counts. Exudative fluid with an elevated leukocyte count was aspirated from the bullae of two patients; one aspirate grew Bacteroides melaninogenicus on culture. Bronchoscopy in seven patients influenced neither the diagnosis nor therapy. Thirteen patients received either oral penicillin (11 patients) or oral tetracycline (two patients). Complete resolution of symptoms and air-fluid levels on chest roentgenography occurred in all patients over two to 32 weeks (mean, 12).