Thymic size can be affected by both exogenous and endogenous glucocorticoids. Development of the respiratory distress syndrome (RDS) is influenced by adrenal cortical function, Thus, thymic size in RDS is considered to be enlarged due to decreased adrenal cortical function. To find whether the presence of RDS correlates with the thymus, the size and shape of the thymus were evaluated in the radiographs of premature infants with RDS, without RDS (control prematurity) and normal infants. The subjects were consisted of chest films of Korean premature infants, 120 with RDS, 60 without RDS, and 60 of normal infants taken at the Department of Radiology, Our Lady of Mercy Hospital during the period of 62 months since January 1978. Relative size of the thymus was determined by cardiothymic/thoracic ratio (CT/T ratio). Grading and location of the thymic prominence as well as incidence of the shape were examined. And all the relations among the radiographs of RDS, control prematurity and notmal infants were analyzed. the results were as follow: 1. The CT/T ratio of premature infants with RDS was significantly greater than that of control prema-turity and normal infants (P<0.01) 2. The incidence of bilateral thymic prominence was more frequent in premature infants with RDS than in control prematurity and normal infants (P<0.05) 3. The frequency of thymic prominence was greater in the right than left side in all the three groups (P<0.05). 4. As in the shape of the thymus, a rounded type was most frequent, and a triangular type was least frequent in ajll the three groups. 5. Incidence of RDS was very low (9.8%) when the CT/T ratio is below 0.3 and it was very high (90.9%) when the CT/T ratio is above 0.49