Background Proteinuria is a marker for underlying diabetic nephropathy and may be a surrogate marker for advanced atherosclerosis. It is unknown if proteinuria is a determinant of death in patients with diabetes after coronary artery bypass grafting. We hypothesized that diabetic patients with evidence of proteinuria would have increased mortality and clinical event rates after isolated coronary artery bypass grafting compared with nonproteinuric diabetic patients. Methods and Results We performed an observational of study of 905 diabetic patients with urinalysis and available follow-up data (nonproteinuria, n = 651; proteinuria, n = 254) after isolated coronary artery bypass grafting at the Cleveland Clinic Foundation between January 1989 and December 1992. The proteinuria group was further prospectively stratified into low-concentration (n = 225) and high-concentration (n = 29) groups. The end points of this study were all-cause mortality and the composite end point of death, nonfatal myocardial infarction, and need for repeat revascularization. The mean follow-up time was 66 months. The 5-year mortality rate for the nonproteinuria and proteinuria groups was 20.2% and 29.1% (P <.001), respectively. The 5-year rate of death, nonfatal myocardial infarction, and need for repeat revascularization for the nonproteinuria and proteinuria groups was 25.2% and 36.2% (P <.001), respectively. Significant multivariate predictors of 5-year mortality included age, not using a left internal mammary artery graft to the left anterior descending coronary artery, proteinuria, lower body weight, and increased creatinine level. Conclusions Among diabetic patients, proteinuria appears to be an important predictor of death after isolated coronary artery bypass grafting. (Am Heart J 2000;139:939-44.)