Objective To explore the value of procalcitonin (PCT) in the prediction of the prognosis and severity of bacterial infection in critically ill patients.Methods A total of 116 eligible patients with bacterial infection admitted in the intensive care unit were enrolled in this prospective study from February,2012 through November,2012.Within 24 hours after admission,the serum PCT was determined with immune-chromatography and acute physiology and chronic health evaluation Ⅱ (APACHE Ⅱ) score of patients was calculated.Based on the 28-day clinical outcome of patients,the patients were divided into fatal group (n =36) and survival group (n =80).The differences in PCT and APACHE Ⅱ score between the two groups were compared with t test or rank-sum test.The correlation between PCT and APACHE Ⅱ score was determined with Spearman's correlation analysis.Both PCT and APACHE Ⅱ score were analyzed separately and jointly with area under receiver operator characteristic curve (ROC curve,AUC) to predict 28-day survival.Comparison of prediction performance for predicting 28-day survival of patients with bacterial infection between PCT and APACHE Ⅱ was made with U test.Results PCT concentration was significantly higher in fatal group than that in survival group [5.36 (2.07,25) vs.0.24 (1.00,2.14)] (Z =5.596,P ＜0.01).APACHE Ⅱ score within 24 hour after admission was significantly higher in fatal group than that in survival group (24.30 ± 6.71) vs.(16.03 ± 7.23),t =6.147,P ＜ 0.01.Positive correlation between PCT and APACHE Ⅱscore was found to be statistical significance (r =0.388,P＜ 0.01).When rates of 28-day survival in patients were predicted by using PCT and APACHE Ⅱ score,the areas of under curve were 0.804 and 0.792,respectively.AUC of PCT was tenuously larger than that of APACHE Ⅱ score (U =0.2073,P =0.802).Using PCT and APACHE Ⅱ score together to predict 28-day survival,AUC (0.817) was increased.The joint prediction performance was higher than that of either alone,increasing the sensitivity to 90.7％ and the specificity to 75.2％.Conclusions Serum PCT can reflect the severity of the illness and prognosis of infectious disease in the intensive care unit.It can serve as a sensitive marker of predicting 28-day survival.Combining PCT and APACHE Ⅱ score together can increase the prognostic value.