To evaluate the effects of transfusions in patients with severe sepsis and septic shock on mortality. Propensity-matched analysis of a prospective observational database (April 2005 to February 2009). Twenty-two medical and surgical intensive care units in 12 teaching hospitals in Korea. One thousand fifty-four patients with community-acquired severe sepsis and septic shock. None. Of the 1,054 patients, 407 (38.6%) received a blood transfusion. The mean pretransfusion hemoglobin level was 7.7 ± 1.2 g/dL. Transfused patients had higher 28-day and in-hospital mortality rates (32.7% vs. 17.3%; p < .001, 41.3% vs. 20.3%; p < .001, respectively) and a longer duration of hospital stay (21 interquartile range, 10-35 vs. 13 interquartile range, 8-24 days; p < .001), but were more severely ill at admission (lower systolic blood pressure, higher Acute Physiology and Chronic Health Evaluation II score, and Sequential Organ Failure Assessment score at admission). In 152 pairs matched according to the propensity score depending on patient transfusion status, transfused patients had a lower risk of 7-day (9.2% vs. 27.0%; p < .001), 28-day (24.3% vs. 38.8%; p = .007), and in-hospital mortality rates (31.6% vs. 42.8%; p = .044). After adjusting for blood transfusion as a time-dependent variable in multivariable analysis, blood transfusion was independently associated with lower risk of 7-day (hazard ratio 0.42, 95% confidence interval 0.19-0.50, p = .026), 28-day (hazard ratio 0.43, 95% confidence interval 0.29-0.62, p < .001), and in-hospital mortality (hazard ratio 0.51, 95% confidence interval 0.39-0.69, p < .001). In this observational study of patients with community-acquired severe sepsis and septic shock, red blood cell transfusions were associated with lower risk of mortality.