Backgound: This study was performed to evaluate the incidences, the risk factors, and the clinical course of the hyperamylasemia in patients who underwent open heart surgery under cardiopulmonary bypass. Material and Method: Thirty seven patients who underwent cardiopulmonary bypass were studied at Department of Thoracic & Cardiovascular Surgery, Yeungnam University Hospital, from July 1997 to June 1998. The thirty seven patients were divided into two groups, 13 patients in group I had normal serum amylase levels and 24 patients in group II had hyperamylasemia. Result: Mean serum amylase(IU/l) levels and 24 patients in group II had hyperamylasemia. Result: Mean serum amylase(IU/l) levels of gorup II showed 54.3$pm$4.6, 78.0$pm$9.2, 372.0$pm$103.4, 460.5$pm$80.4, 280.4$pm$46.6, and 131.0$pm$15.6, preoperative, immediate postoperative, at postoperative 1, 2, 3, and 7 days, respectively. In group II, serum amylase level of the postoperative day 2 was the highest and was significantly higher than that of the preoperative day(p<0.001). Serum amylase level started to decreased at postoperative day 3 and returned to the normal level at postoperative day 7. Significant clinical symtoms of overt pancreatitis were not shown in patients in group II. The following perioperative variable such as diagnosis, cardiopulmonary bypass time, aortic cross clamping time, mean systemic pressure during bypass, and administration of steroid were compared between groups. There were no significant differences between groups. In all patients, Serum amylase level of postoperative day 2 and aortic cross clamping time were correlated significantly(p=0.047). Conclusion: Serum amylase level after cardiopulmonary bypass could be elevated postoperatively and serum amylase level of POD 2 was considered to have significant correlation with aortic cross clamping time. Shortening of aortic cross clamping time will help in reducing the hyperamylsemia. In this study, although significant clinical symptoms and overt pancreatitis were not seen from hyperamylsemic patients, careful clinical observation of hyperamylasemia would be necessary.