OBJECTIVE: To investigate the significance of CD11b expression in neutrophils and lymphocytes in children with systemic inflammatory response syndrome (SIRS). METHODS: CD11b expression in neutrophils and lymphocytes was measured using flow cytometry in 36 children with SIRS (SIRS group) and 28 children with infectious disease but without SIRS (control group). The sensitivity and specificity of neutrophil CD11b for diagnosis of SIRS were evaluated. RESULTS: During the acute phase, an increased CD11b expression in neutrophils (96.7±8.1%) was observed in the SIRS group compared with the control group (85.1±5.1%) (P<0.05). Using neutrophil CD11b expression >92.2% as a cut-off value for diagnosis of SIRS, the sensitivity and the specificity were 97.2 % and 92.9% respectively. Lymphocytic CD11b expression in the SIRS group (13.4±8.6%) was lower than that in the control group (19.2±6.4%) in the acute phase (P<0.05). In the SIRS group, lymphocytic CD11b expression was remarkably suppressed in the severe sepsis subgroup (7.27±3.04%), showing significantly decreased expression compared with the non-infectious subgroup (19.3±2.9%) and the sepsis subgroup (15.9±12.5%) (P<0.01). In the convalescence stage lymphocytic CD11b expression in the SIRS group was similar to that in the control group. CONCLUSIONS: CD11b expression in neutrophils may serve as a reliable indicator for diagnosis of SIRS. The down-regulation of lymphocytic CD11b expression might be a signal of the condition aggravation in children with SIRS.［Chin J Contemp Pediatr, 2009, 11 (7):540-542］