A retrospective double blind study comparing 7 day with 2 day regimen of antibiotic prophylaxis was conducted among 200 patients undergoing open heart surgery. No case of endocarditis and wound infection occurred. Pneumonia developed in 5 cases of the 7 day and 1 case of the 2 day group. Urinary tract infection without clinical significance developed in 1 case of the 7 day and 3 cases of the 2 day group. Bacteremia developed in 2 cases of the 7 day and 1 case of the 2 day group. We concluded as follows: l. Administration of antibiotics for 2 days appears to be without substantial risk of infection comparing long term 7 day regimen. 2. 7 days of antibiotics may actually increase the risk of serious infection such as nosocomial pneumonia, and predispose to the development of infections with fungi or antibiotic resistant bacteria. 3. 2 days of prophylaxis is more beneficial than long term 7 day regimen for example economically.