Cardiac measurement is essential in the evaluation of the hart disease. In a simple chest roentgenogram, cardiomegaly may be determined by measuring the transverse cardiac diameter, cardio-thoracic ratio, frontal cardiac area and the heart volume. The size of the heart may be most accurately assessed by calculation the heart volume. However, it is time-consuming and of little practical value in a busy department. Measuring the transverse cardiac diameter and cardio-thoracic ratio is simplest and convenient although admittedly it is less accurate than the volumetry. In an attempt to determine the mean and normal upper limits of the transverse cardiac diameter in the healthy Korean adults, the present study was carried out. The measurement was made by 2 different methods. In group A, the transverse cardiac diameter was measured including the pericardial fat pad and the internal thoracic diameter was measured at the level of the highest point of the right hemidiaphragm (399 cases . In group B. the transverse cardiac diameter was measured at the level of the widest internal diameter of the thorax (411 cases). The results are as follows: 1. The mean of the transeverse cardiac diameter is 12.6$pm$1.2cm in male and 11.8$pm$1.0cm in female when the pericardial fat pad is included. The mean of the transverse cardiac diameter is 12.5$pm$1.1cm in male and 11.6$pm$1.0cm in female when the pericardial fat pad is excluded. The difference is significant in female (P＜0.05). 2. When the pericardial fat pad is included in the transverse cardiac diameter, the upper limit of the transverse cardiac diameter is 15.0cm in most normal male and 13.9cm in female. Therefore, a transverse cardiac diameter larger than 15.5cm in male and 14.4cm in female may be considered abnormal. 3. When the pericardial fat pad is excluded the upper limit of the transverse cardiac diameter is 14.8cm in most normal and 13.5cm in female. Therefore, a transverse cardiac diameter larger than 15.3cm in male and 4.0cm in female may be considered abnormal. 4. The mean of cardio-thoracic ratio is 43.2$pm$3.9% in male and 44.4$pm$3.5% in female when the pericardial fat pad is excluded.