PURPOSE: We wanted to evaluate the incidence and clinical features of perifissural air seen in the pneumomediastinum on CT. MATERIALS AND METHODS: The study included the CT scans of 90 patients with pneumomediastinum (65 males and 25 females) and who had been treated during the recent 3 years. The ages ranged from 2 to 48 years (mean age: 41 years). The patients were divided into 3 groups, I: perifissural air (n=7), II: pneumomediastinum without perifissural air (n=83) and III: pulmonary interstitial emphysema involving the peribronchial or intraparenchymal area (n=9) of the Group II. We analyzed the clinical data and compared it between group I and the other groups. RESULTS: Group I included 7.8% of the total cases. Age was statistically different between Group I and II, but age was not significantly different between Groups I and III (I: 13.7 years, II: 42.9, III: 26.4). A spontaneous cause was the most common (57.2%) in Group I, trauma (33.7%), postoperative occurrence (20.5%) and a spontaneous cause (9.7%) were common in Group II, and trauma (44.4%) and a spontaneous cause (33.3%) were common in Group III. The incidence of a spontaneous cause was statistically different between Groups I and II (p<0.001), but this was not significantly different between Groups I and III. CONCLUSION: The incidence of perifissural air seen in the pneumomediastinum on CT is 7.8%. Perifissural air is more common in the younger patients with a spontaneous cause.