Thymic size can be affected by both exogenous and endogenous glucocorticoids. Development of the respiratorydistress syndrom (RDS) is influenced by adernal cortical function. thus, thymic size in RDS is considered to beenlarged due to decreased adrenal cortical function. To find whether the presence of RDS correlates with thethymus, 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 prematureinfants, 120 with RDS, 60 without RDS, and 60 of normal infants taken at the Departement of Radiology, Our Lady ofMercy Hospital during the period of 62 months since January 1978. Relative size of the thymus was determined bycardiothymic/thoracic ratio(CT/T ratio). Grading and locaiton of the thymic prominence as well as incidence of theshape were examined. And all the relations among the radiographs of RDS, control prematurity and normal infantswere analyzed. The results were as follows; 1. The CT/T ratio of premature infants with RDS was significantlygreater than that of control prematurity and normal infants (P<0.01). 2. The incidence of bilateral thymicprominence 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 allthe three groups. 5. Incedence 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.