Objective: The first influenza virus pandemic of the 21st century began in Mexico in 2009 and spread rapidly all over the world. We described the clinical and epidemiologic characteristics of the 31 patients admitted to the Deparment of Chest Disease due to influenza A (H1N1) virus infection. Material and Method: In this study we evaluated 51 patients with high clinical suspicion for Influenza A (H1N1) virus infection. We used real time reverse transcriptase chain reaction test (RT-PCR) test for diagnosis. Thirty-one (15 female, 16 male) of 51 patients RT-PCR test were positive for Influenza A (H1N1) virus infection. Demographic features, clinical and laboratory characteristics of these patients were assessed.Results: The mean age of Influenza A (H1N1) virus infected patients was 40.8±13.1. Cough was the most common symptom (90%) and 11 patients (35%) had pneumonia. Leucopenia (48.3%), C reactive protein elevation (90.3%) and creatine phosphokinase elevation (71%) were notable laboratory findings. T and B lymphocyte subgroups were also evaluated and compared with Turkish adult normals. Lymphocyte subgroups were not different in the patients with influenza A (H1N1) virus statistically. Conclusion: Influenza virus may cause pandemias with different subgroups. Although most Influenza A (H1N1) virus infected patients hospitalized in our clinic had progressed well but pulmonary complications should be considered.