PURPOSE: Thymus size can be affected by several factors and perinatal diseases can be estimated by its size. The purpose of this study was to search for a relationship between cardiothymic/ thoracic(CT/T) ratio and perinatal diseases such as neonatal respiratory distress syndrome(RDS) and intrauterine growth retardation(IUGR) by measuring the width of the cardiothymic shadow at the level of the carina and dividing it by the width of the thorax at the costophrenic angles. METHODS: A clinical study was conducted on newborn infants with RDS(n=51), IUGR(n=27), and premature rupture of membranes(PROM, n=48), who were admitted at NICU of Catholic University of Daegu from June 2000 to Oct. 2001. CT/T ratio was measured within six hrs of age, at 2-3 days of age, and at 5-7 days of age. RESULTS: CT/T ratios of RDS group, IUGR group, and PROM group were 0.46+/-0.07, 0.32+/-0.04. 0.36+/-0.06, respectively. CT/T ratios of RDS group within 6hrs of age, at 2-3 days of age, and at 5-7 days of age were 0.43+/-0.07, 0.34+/-0.06, 0.25+/-0.04, respectively. There were statistically significances among the RDS group, the IUGR group, and the PROM group and in the RDS group at different times. Regression for gestational age among three groups was not statistically significant but correlation for gestational age in the entire groups was statistically significant. CT/T ratio between normal spontaneous vaginal delivery and c-section among three groups was not statistically significant. CT/T ratios with dexamethasone-treated group and untreated group was not statistically significant. CONCLUSION: We concluded that thymus size differed significantly in the perinatal diseases such as RDS and IUGR, and so can be used as an early diagnostic tool for perinatal diseases.