The changes in hematocrit during the intra and postoperative period were studied in 50 patients who underwent uncomplicated open-heart procedures and who did not receive blood or blood products during their hospital stay. Priming of the extracorporeal circuit and blood volume replacement postoperatively was done with crystalloid solutions exclusively. Mean preoperative hematocrit was 44% and dropped to 30.8% during the pump run. Lowest postoperative hematocrit was on the fourth day (mean: 29.3%) but it never went below 50% of the preoperative value. The percentage of the initial hematocrit was calculated for all patients at each time point and a median curve was constructed. Then, the curve of each patient was compared to this median curve. Patients with values below the median curve at one point at least had a larger amount of intra and postoperative fluid replacement than patients that stayed above the median at all times. We conclude that non-transfused open-heart surgical patients do not develop severe anemia and that changes in the hematocrit values can be predicted according to the median curve constructed and to the amount of crystalloid solution given during and after surgery.