The aim of this study was to evaluate the characteristics and CT features of subcarinal air collections on thin-section multidetector-row computed tomography (MDCT). Two hundred asymptomatic adult subjects without a history of pulmonary disease underwent MDCT. The CT appearances and characteristics of foci of extraluminal air contiguous to the main bronchus in the subcarinal region were retrospectively analysed. Subcarinal air collections were found in 81 of 200 subjects (41%) and were spotty or microtubular in 67 of 81 subjects and rounded or oval in the other 14. Each subcarinal air focus communicated, or seemed to communicate, with the adjacent bronchus in 76 of 81 cases. Our data demonstrate that extraluminal air foci in the subcarinal region are common CT findings and routinely depicted on thin-section MDCT, and that most of these lesions seem to be main bronchial diverticula. Cystic air foci in the subcarinal region should be called subcarinal air cysts. The precise recognition of these cysts improves the ability to avoid misidentification, such as pneumomediastinum.