Background and Objectives: Preoperative renal dysfunction is a risk factor for postoperative morbidity and mortality after coronary artery bypass graft (CABG). However, the impact of preoperative renal dysfunction on valve replacement outcome is not well known. The objective of this study was to investigate the effect of preoperative mild renal dysfunction not requiring dialysis on mortality and morbidity after cardiac valve surgery. Subjects and Methods: Between November 2003 and July 2007, clinical data were collected for 121 patients who had cardiac valve surgery with or without CABG. Preoperative renal dysfunction was calculated with the abbreviated Modification of Diet in Renal Disease formula and was defined as a glomerular filtration rate (GFR). Logistic regression analysis was used to assess the effect of preoperative renal dysfunction on operative and adverse outcomes. Results: Eighteen patients (14.9 %) had preoperative mild renal dysfunction. Patients with preoperative mild renal dysfunction showed more co-morbidities but did not show significantly higher rate of preoperative anemia (30.1% vs. 50.0%, P=0.098). Patients with preoperative mild renal dysfunction had worse postoperative outcomes with decreased left ventricular ejection fraction (OR, 5.27; 95% CI, 1.08-25.83; p=0.040) and post-operative atrioventricular block (OR, 10.72; 95% CI, 1.25-91.78; p=0.030). Preoperative mild renal dysfunction was significantly and independently associated with all major post-operative complications(OR, 3.11; 95% CI, 1.01-9.55; p=0.048). But mortality was similar in both groups (0% vs. 5.6%, P=0.149) regardless of preoperative mild renal dysfunction. Conclusion: Preoperative mild renal dysfunction in patients undergoing cardiac valve surgery is an independent predictor of postoperative morbidity.