With the introduction of new cardiac prosthesis, it behooves surgeons and cardiologists to monitor its performance carefully. ATS (Advancing The Standard) prosthetic valve has been used first in Guro hospital in Korea, since August 1994. Between August 1994 and July 1995, 21 patients received 28 ATS prosthesis(9 aortic, 19 mitral).19mi1ra1 valves were implanted through the "Extended Transseptal Approach" 10 were ma e and 11 were female, ranging from 20 to 54 years of age(Mean age : 37 years). The follow up period 126 patient-months(mean 6.1 months), varied from 1 month to 12 months. NYHA functional class was improved significantly, from $2.9pm0.7$ preoperatively to $1.4pm0.5$ postoperatively. Ejection fraction was also improved from $55.5pm6.1%$ preoperatively to 59.8 $pm7.4%$ postoperatively. Lactic dehydrogenase(LDH) was used as an indicator of hemolysis. The value of LDH changed from 483.3 $pm$ 162 lUlL preoperatively to $527pm274$ lUff postoperatively with no clinical significailce. Valve related complications, such as thromboembolism, valve thrombosis, anticoagulant related hemorrhage and prosthetic valve endocarditis did not develop except one anticoagulant related intracranial hemorrhage. There were no mortalities. This experience encourages us to continue using the ATS prosthetic valve, and this study will help those patients who need to have their heart valves replaced. replaced.