Abstract: Objective To analyze epidemiological characteristics of pandemic influenza A (H1N1) in Shanghai and provide scientific evidence for the further improvement of the control and prevention measures. Methods Throat swabs of the patients were taken to detect nucleic acid of pandemic influenza A (H1N1) virus. Descriptive epidemiological analysis and related risk factor analysis were conducted on the pandemic influenza A (H1N1) cases confirmed in Shanghai during May-September. Results The first case of pandemic influenza A (H1N1) in Shanghai was confirmed on May 25, 2009, and by 24 pm, November 19, 2009, 1656 confirmed cases had been reported in Shanghai, including 4 severe cases, without death. Epidemiological study was performed on 315 confirmed cases reported initially. The vast majority (296, 93.97%) were imported cases, 139 cases were from Australia (44.13%) and 43 from United States (13.65%). Since June 29, the reported cases had increased substantially. Most cases were directly identified by following 4 ways: influenza surveillance system, the port quarantine, febrile disease clinic screening and centralized medical observation. Most cases were adolescents and young adults (64.73%). There was gender difference on incidence (P=0.003). A total of 5224 close contacts were followed up, including 20 health care workers. No infection occurred among health care. workers. Conclusion The epidemic of pandemic influenza A (H1N1) is in rapid spread and there is more potential that the outbreak of pandemic influenza A (H1N1) might occur in the community in Shanghai.