Objective: Hypophosphatemia can cause a chain of chemical reactions leading to acute respiratory failure. Therefore it is of potential with regard torelevance with postoperative respiratory complications. The aim of our study was to evaluate the correlation between the serum phosphate level and pulmonary functions after cardiac surgery. Material: A total of 66 patients who had a normal initial phosphate level and who had cardiac surgery under cardiopulmonary bypass in a tertiary healthcare hospital between November 2013 and May 2014 were enrolled in our study. All of their data was retrospectively evaluated. Method: All the operations were performed by the same surgical team. Patients were grouped according to postoperative serum phosphate levels as group A with normal level and group B with low level. Results: The demographic and pre-operative variables were found to be similar between both groups (p > 0.05). Intraoperative and postoperative comparisons of the two groups showed that the postoperative phosphate level (p = 0.001) and postoperative use of inotropic agent (p = 0.047) differed significantly. Correlation analyses showed a significant negative correlation between postoperative phosphate levels and time of mechanical ventilatory support (r = -0.367; r(2) = 0.135; p = 0.002). Conclusions: Hypophosphatemia was found to be an independent risk factor for prolonged mechanical ventilatory support. Postoperative measuring of phosphate serum levels should not be underestimated after cardiac surgery.