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Background: The prevalence of obesity in most developed nations, including Australia, continues to rise and represent an increasing public health concern. Obesity has been considered a major risk factor in patients undergoing cardiac and other major surgery., Methods: We retrospectively analysed prospectively collected data of consecutive patients undergoing cardiac surgery between June 2001 and February 2006 at two Australian public hospitals. Patients were divided into three groups by body mass index (BMI): non-obese (BMI 20-30), obese (BMI>30-40) and morbidly obese (BMI>40). Associations between early mortality and morbidity and obesity were assessed by univariate and multivariate methods., Results: Out of 4053 patients, 85 were excluded for BMI<20. A total of 2743 patients were defined as non-obese, 1136 obese and 89 morbidly obese. There were no significant differences in operative mortality, stroke, pneumonia, new renal failure, atrial fibrillation, prolonged ventilation, reintubation, readmission to intensive care, prolonged length of hospital stay or readmission within 30 days. The morbidly obese group had increased rates of deep sternal infection by univariate (odds ratio [OR] 6.4, 95% confidence interval [CI] 2.1-19.1, p<0.001) and multivariate (OR 13.1, CI 3.4-50.7, p<0.001) analysis. The obese group had a lower rate of re-operation for bleeding by univariate (OR 0.61, CI 0.41-0.91, p=0.01) and multivariate (OR 0.64, CI 0.42-0.99, p=0.04) analysis., Conclusion: Apart from an increased rate of deep sternal wound infection, obesity is not associated with early mortality or other post-operative complications. The protective effect of obesity on re-operation for bleeding requires further study.