Abstract: Objective:To investigate the correlation between serum procalcitonin(PCT)level in children with or without bacterial infection and clinical severity of disease.Methods:One hundred and seventy-nine children were divided into bacterial infection group(97 cases),virus infection group(59 cases)and uninfection group(23 cases).The levels of PCT in three groups were compared.Fifty-two cases in bacterial infection group who accorded wih the diagnostic criteria of systemic inflammatory response syndrome were assessed with pediatric critical illness score(PCIS).The correlation between PCT level and severity of clinical infection was observed.Results:The level of PCT in bacterial infection group was significantly higher than those in virus infection and uninfection group(P0.01).As the critical value for 0.5 ng/ml,the sensitivity and specificity of PCT in the diagnosis of bacterial infection were 80.41% and 82.93%,respectively.As the critical value for 2.0 ng/ml,the sensitivity and specificity of PCT in the diagnosis of bacterial infection were 30.93% and 98.78%,respectively.PCT level increased with PCIS scores decreasing.PCIS score was negatively correlated with PCT level,and positively correlated with the clinical severity of the disease(P0.01).Conclusions:PCT can be used to diagnose the early bacterial and non-bacterial infection,guide the rational use of antibiotics,warn whole body serious infection,guide clinical treatment and determine the prognosis of the disease.