According to the preoperative phosphate levels of the 78 patients in the study group, 6(7.69%) of them were hypophosphatemic. In the postoperative period, 41 patients (52.57%) were hypophosphatemic (p<0.01). The patients in the hypophosphatemic group were ventilated for 9.8±1.9 hours postoperatively, where as patients in the control group requiired 7.3±1.2 hours mechanical ventilation (p<0.05) We conclude that hypophosphatemia is common after open heart surgery in the immediate postoperative period, and it may be responsible for the problems seen in the postoperative respiratory treatment course.