['Anemia is an established adverse risk factor in cardiovascular disease. However, the effect of preoperative anemia is not well defined in heart surgery. This study evaluates the effect of preoperative anemia on early clinical outcomes in patients undergoing cardiac surgery.', 'A retrospective, observational, cohort study of prospectively collected data was undertaken on 7,738 consecutive patients undergoing heart surgery between April 2003 and February 2009. Of these, 1,856 patients with preoperative anemia were compared to 5,882 patients without anemia (control group). According to the World Health Organization, anemia was defined as hemoglobin level\u2009<\u200913 g/dl for men and <12 g/dl for women. Selection bias not controlled by multivariable methods was assessed with propensity-adjustment method.', 'Overall mortality was 2.1%. Preoperative anemia was associated with tripling in the risk of death (4.6% vs 1.5%, p\u2009<\u20090.0001) and postoperative renal dysfunction (18.5% vs 6.5%, p\u2009<\u20090.0001). There was also a significant difference between the anemic and non-anemic group in the risk of postoperative stroke (1.9% vs 1.1%, p\u2009=\u20090.008), atrial fibrillation (36.7% vs 33%, p\u2009=\u20090.003) and length of hospital stay\u2009>\u20097 days (54% vs 36.7%, p\u2009<\u20090.0001). In propensity-adjusted, multivariable logistic regression, preoperative anemia was an independent predictor of mortality (odds ratio [OR] 1.44, 95% confidence interval [CI] 1.02 to 2.03), postoperative renal dysfunction (OR 1.73, 95% CI 1.43 to 2.1) and length of hospital stay\u2009>\u20097 days (OR 1.3, 95% CI 1.15 to 1.47).'] Does preoperative anemia increase mortality and postoperative morbidity after cardiac surgery?