The aim of this study is to evaluate the effect of gender and age on outcome following coronary surgery in several hospitals enrolled in a national quality assessment program. Patients undergoing isolated coronary artery bypass graft (CABG) during 2003-2005 in Italy were included in the study (n=74,577). The outcome measure was 30-day in-hospital mortality. Comorbidities recorded during previous and current hospitalizations were used to define patients' health status and to build the adjustment model. The interaction term (gender*hospital) was introduced into the model to test the effect modification of gender; if present, gender specific models were analyzed to test the effect modification of age. A significant effect modification by gender was found in 39 hospitals; the adjusted odds ratios (AdjORs) showed significant increased risk for females (AdjORs ranging from 3.7 to 21.6). In three of these hospitals a significant increased risk was found for older age (AdjORs for elderly patients ranging from 8.1 to 14.6). Two hospitals showed a significant excess risk for patients ≥75 years (AdjORs=6.6 and 13.8). The technical aspects of surgery could account for the excess risk found in female patients; differences in the entire care process (intraoperative and postoperative management) could explain the variations in outcome among elderly patients.