Abstract: Objective To study the changes of serum complement components C3, C4 in patients with H1N1 influenza A. Methods Serum C3, C4 levels and serum CH50 level were measured by turbindimetric immunoassay（TIA） and liposome immunoassay, respectively, in 369 patients with mild H1N1 influenza A（group A） and 23 patients with severe H1N1 influenza A（group B） during the initial 5 days after admission. Results The mean level of C3 was 1.10±0.18g/L and 1.10±0.19g/L, respectively, while the mean level of C4 was 0.29±0.08g/L and 0.29±0.08g/L, respectively, in groups A and B. The CH50 level was either slightly elevated（76%） or significantly decreased（15%）. The mean level of CH50 was 48.84±19.38U/ml and 57.48±13.25U/ml, respectively in groups A and B（P=0.036）. Conclusion Serum C3 and C4 levels are higher than normal and CH50 level is either slightly elevated or significantly decreased in patients with H1N1 influenza A at its early stage. The CH50 level is higher in patients with severe H1N1 influenza A than in those with mild H1N1 influenza A, which is of certain significance for the early warning of severe H1N1 influenza A.