OBJECTIVE: To study the relationship between the changes of inflammation-associated factors, C-reactive protein (CRP), procalcitonin (PCT), erythrocyte sedimentation rate (ESR), white blood cell (WBC) and neutrophils, and the severity in children with Mycoplasma pneomoniae pneumonia (MPP). METHODS: Ninety-two children with acute MPP consisting of 52 cases with concomitant systemic inflammation response syndrome (SIRS) and 40 cases without SIRS were enrolled in this study. The 52 cases with concomitant SIRS were classified into two groups based on the severity of SIRS: mild SIRS (n= 25) and severe SIRS (n=27). CRP, PCT, ESR and WBC count and the percentage of neutrophils (NE%) were detected on admission and one week after anti-inflammation treatment. RESULTS: All of patients showed increased serum CRP contents at admission. The serum CRP contents were the highest in the severe SIRS group, followed by the mild SIRS and non-SIRS groups on admission (P< 0.05 or 0.01). The serum CRP contents were reduced in all of patients after 1-week treatment. The severe SIRS group still demonstrated higher serum CRP contents than the non-SIRS and the mild SIRS groups (P< 0.01). The severe SIRS group had increased serum PCT contents on admission, which were significantly higher than those of the mild SIRS and non-SIRS groups (P< 0.01). After 1-week treatment, the serum PCT contents were reduced in the severe SIRS group but remained higher than in the mild SIRS and non-SIRS groups (P<0.01). ESR increased significantly in the severe SIRS group than in the mild SIRS and non-SIRS groups on admission (P < 0.01). One-week treatment did not significantly decrease ESR in all three groups. The WBC count and NE% in the mild and severe SIRS groups were significantly higher than in the non-SIRS group and the severe SIRS group had higher WBC count and NE% than the mild SIRS group on admission (P<0.05). The WBC count and NE% decreased after 1-week treatment in the mild and severe SIRS groups (P<0.05). One inflammation-associated factor (only CRP) increase was predominant in the non-SIRS group (65%), 2 factors increase in the mild SIRS group (56%), and three or more factors increase in the severe SIRS group (70.4%). CONCLUSIONS: The detection of inflammation-associated factors, CRP, PCT, ESR, WBC and neutrophils, are valuable to the evaluation of severity in MPP.［Chin J Contemp Pediatr, 2007, 9 (4):347-350］