Objective:To evaluate the risk factors for severe cases of influenza A H1N1. Methods:The confirmed cases of influenza A H1N1 during June to December 2009 in Jiangsu province were investigated using unified designed questionnaires. Univariate and multivariate non-conditional logistic regression were employed to analyze the risk factors and odds ratio(OR)with SAS 9.1.3. Results:Age,interval between incidence and first-visit,BMI,pregnant and chronic comorbid illness were identified as the risk factors of severe cases in univariate logistic regression models. And age,interval between incidence and first-visit,BMI and chronic comorbid illness got into the final multivariate logistic model. Among which,the contribution of chronic comorbid illness to the model was greatest(β=0.72),followed with age(β=0.43),BMI(β=0.35)and interval between incidence and first-visit(β=0.25). Joint effect analysis indicated a dose-response relationship between the increased risk and the amount of risk factors. Conclusion:Individuals with risk factors for severe cases of infulenza A H1N1 should receive earlytreatment.