Abstract: Objective To investigate the antibody level to 2009 pandemic influenza A (H1N1) virus in out-patients and healthy people in Chongqing, analyze/predict the epidemic trend and provide evidence for the prevention and control of the disease. Methods Seroepidemiological survey was conducted among 238 out-patients and 171 healthy people by taken blood samples from them to detect 2009 pandemic influenza A (H1N1) virus with hemagglutination inhibition (HI) test. Results The total positive rate of HI test was 15.4% (95% CI: 12.1%-9.4%), and the people with GMT 1 ∶ 160 accounted for 6.4% of the total (95％ CI: 0.4%-19.5%). The differences on GMRT between males and females, the people with or without influenza like symptom 2 weeks ago, age group of5 years and the other 3 age groups as well as the people receiving the vaccination or not had statistical significance (P0.05). The HI positivity rate, titer distribution and logistic regression indicated that the influenza like symptom 2 weeks ago was the related factors, but the differences between males and females, and among age groups had no statistical significance. Conclusion The antibody level to 2009 pandemic influenza A (H1N1) virus was low in Chongqing, and the epidemic of 2009 pandemic influenza A(H1N1) might occur.