Objectives: The first objective of this study is to evaluate the incidence of postoperative renal dysfunction (PRD) and its impact on perioperative mortality and morbidity on hospital length of stay. The second objective is to differentiate between a normal and a mildly elevated serum creatinine level (1.3–2.0 mg/dl) on perioperative mortality and morbidity. Methods: This was a prospective, comparative, non-randomized, analytic study that enrolled a total of 50 patients who had no significant pre-existing renal disease (creatinine 2.0 mg/dl and without dialysis) and who underwent isolated coronary surgery under cardiopulmonary bypass (CPB) in Al-Kasr Al-Ainy University Hospital, Cairo University were recruited to the study during the time from July 2011till January 2012. Results: Multivariable logistic regression identified that, PRD was more likely in diabetics (p= 0.041, OR= 4.23), patients with pre-existing renal dysfunction (p < 0.0001), patients who had a low urine output on bypass (p = 0.021), patients out of the OR on noradrenaline (p= 0.022), and those who received banked blood units( p = 0.17). In group 1, the average length of postoperative ICU stay was 3.56 ± 1.53 days and the average total hospital stay was 9 ± 3.096 days while in group 2 the average length of postoperative ICU stay was 6.28 ± 6.017 days and the average total hospital stay was 12.64 ± 6.389 days. Conclusion: The most likely cause of PRD is a state of pre-existing renal dysfunction. Patients with mildly elevated serum creatinine (1.3- 2.0 mg/dl) have higher morbidities, most notably increased length of ICU and total in-hospital stay.