Abstract: Objective To study the serum procalcionin（PCT） level in diagnosis of secondary infection in patients with severe craniocerebral injury. Methods Serum PCT level,white blood cells（WBC） and C-reaction protein（CRP） level were observed in 31 patients with secondary infection of severe craniocerebral injury（infection group）,and 30 patients without secondary infection of severe craniocerebral injury（non-infection group）,and 30 healthy controls（control group）.Relation between dynamic serum PCT level change and prognosis was studied in an additional 6 patients with secondary infection of severe craniocerebral injury. Results The serum PCT level was higher in infection group than in non-infection group and control group on day 1 after admission（P＜0.01）.No significant difference was found in CRP level between infection group and non-infection group（P＞0.05）,but the CRP level was significantly higher in infection group and non-infection group than in control group（P＜0.001）.No significant difference was observed in WBC between infection group and non-infection group（P＞0.05）,but the number of WBC was greater in infection group and non-infection group than in control group（P＜0.01）.Of the 6 patients with secondary infection of severe craniocerebral injury,3 with their serum PCT level ＞7.3ng/ml died,and 3 survived with their serum PCT level decreased due to the improved clinical symptoms. Conclusion Serum PCT level,which is related with the severity of clinical infection symptoms,plays an important role in diagnosis of secondary infection of severe craniocerebral injury and can also be used as a predictor for the secondary infection in patients with severe brain injury.