Introduction: Influenza pandemic H1N1 is an acute respiratory infectious disease that is combination of two types of influenza virus type A (H1N1). This study aimed to identify risk factors affecting influenza pandemic H1N1. Methods: In this case-control study, the cases were 18 positive cases of pandemic influenza H1N1 and the controls were the patients who were admitted during the same time as the cases to sections of Orthopedics, Urology, Surgery and Women of the same hospital for reasons other than influenza. The data were collected through a form by two experienced nurses and then were fed into SPSS, and were analyzed using independent T-test and chi-square. Results: A significant relationship was observed between pandemic H1N1 influenza infection and a history of domestic travel, contact with confirmed patients, respiratory diseases, and diabetes (P0.05). Conclusion: People with underlying diseases, especially respiratory diseases, diabetes, heart disease and a secondary infection and cardiovascular disease most likely are susceptible to influenza pandemic H1N1.