PURPOSE: To evaluate the frequency of a paratracheal cyst on CT in an adult, and to compare the degree of loculation and the patient's age with the longest diameter of the air cysts. MATERIALS AND METHODS: Of 1520 patients, who underwent a CT scan, 41 patients with paratracheal cysts were enrolled in this study. There were 26-males and 15 females, whose ranged from 24 to 82 years (mean, 59.8). The CT findings were evaluated to determine the relationship between the degree of loculation and the longest diameter of the air cysts. Another tracheal diverticula or an air cyst, as well as the tracheal communication were also evaluated. RESULTS: The longest diameter of the paratracheal air cysts was < 1 cm (n=20), 1-2 cm (n=15), 2-3 cm (n=4), 3-4 cm (n=1) and 4-5 cm (n=1). Most of the patients under 60 years of age had air cysts < 1 cm, and most patients over 60 had air cyst > 1 cm (p=0.043). The paratracheal air cysts < 1 cm were unilocular or bilocular in shape, the 1-2 cm sized air cysts were bilocular or multilocular, and the air cysts > 2 cm were multilocular. Four patients had another small tracheal diverticula, and one patient had another small paratracheal air cyst. Tracheal communication was observed in 7 patients (17%). CONCLUSION: The frequency of paratracheal air cysts in adults undergoing a CT scan was 2.7%. The longest diameter of the paratracheal air cysts was associated with the patient's age. The shape of air cysts became more multilocular as the longest diameter of the paratracheal air cysts increased. Another small tracheal diverticula or air cysts were observed in 12% of patients.