Pulmonary tuberculosis has long been associated with the formation of cavities in the lung. Many reports in the literature indicate that the occurrence of air-fluid levels in tuberculosis is unusual. The authors present 18 [human] cases of proved active cavitary tuberculosis where air-fluid levels occurred during the active phase of the disease. The consideration of tuberculosis in the differential diagnosis of an air-fluid containing pulmonary cavity is emphasized and the pathogenesis of cavity formation is discussed.