Cardio- pulmonary bypass affects the homeostatic system of the body through the effect of heparinization, hemodilution and hypothermia,which represent the major challenges on the homeostatic system.Other factors, like pre-operative aspirin use, old age and CPB time, also affect the homeostatic system. The aim of the study is to find factors which are related to excessive blood loss following open-heart surgery, and to study blood transfusion practice in these patients. The importance of FB transfusion, especially autologous blood, in open-heart surgery was also studied. Forty-eight adult patients, who underwent open-heart surgery over the period of 3 months, were studied. Data were collected from these patients to study their effects on the degree of post-operative blood loss. Thirteen patients (27%) had excessive blood loss (blood loss of >= 1000 ml within the first 24 hours post-operatively). The incidence of excessive blood loss was related to the number of units of FB which were given to the patients post-operatively. It was also found that blood loss can be related to patient’s age, CPB bypass time, type of surgery and pre-operative aspirin use.Two complications of blood transfusion were noted; allergic and pyrogenic reactions .