Decrease of patient’s hematocrit level is a natural process which occurs during the initiation of cardiopulmonary bypass. To some extent it is a beneficial occurrence, but excessive hematocrit decrease can lead to many complications and increase patient’s demand for blood transfusion. In this study authors investigated the causes and clinical implications of ≥≥30% hematocrit decrease after the initiation of cardiopulmonary bypass. Retrospective analysis of 172 patients undergoing elective cardiac surgery procedures was conducted. It was concluded that 30% or greater hematocrit decrease (comparing to preoperative level) after the initiation of cardiopulmonary bypass, is associated with an increased demand for blood transfusion. The most important factors influencing hematocrit decrease after the initiation of cardiopulmonary bypass proved to be: lower BMI, female gender, more advanced age and the usage of priming fluid of higher volume and lower osmolality.