Background & Objectives: Considering new pandemic attack with new emerging (H1N1) influenza virus, the study was designed for evaluating clinical and epidemiologic characteristics of patients in Imam Khomeini hospital, admitted with presumptive diagnosis of H1N1 influenza . In all of the patients clinical and paraclinical findings and outcome (including mortality rate and definitive diagnosis) were evaluated. Bed occupancy rate in infectious disease ward and ICU and also mean days of admission were calculated.

  Methods: This is a cross sectional study . All 118 patients with acute respiratory symptoms and possible diagnosis of emerging H1N1 influenza that had been admitted at least 24 hours in hospital from 20 October to 1 February 2009 were enrolled in the study. Data collection was done based on questionnaires, with a team other than researchers. The questionnaire included demographic data , clinical symptoms , laboratory findings , radiographic manifestations and outcome of patients. Data analysis was performed with SPSS software version16.

  Results: A total of 118 patients were studied: 71 patients ( 60.2%) were female and 47 patients ( 39.8%) were male. Mean ( ± SD) age of admitted patients was 33.81 ± 15.64 years old. The most of admitted patients were in age range of 15 to 30 years old. The most common findings in CXR were bilateral respiratory consolidations and the most common symptoms were fever, weakness and fatigue. About 12.7% of patients had diarrhea. Leukopenia (WBC < 4000) and leukocytosis (WBC > 10000) occurred respectively in 4.58% and 33.2% of cases . Nine patients (7%) were admitted in ICU. I n 21 patients (18%) RT-PCR test results were positive and three of these patients had been admitted in ICU. In patients admitted in ICU while their diagnosis was confirmed, mortality was 33%. 48.3% of patients had at least one predisposing medical condition . Total admission days were 577 days, consisting 519 days in infectious disease ward and 58 days in ICU. Average of hospitalization days per a patient was 4.88 days.

  Conclusion: Except epidemiologic clues, there was no pathognomonic sign or symptom to differentiate influenza H1N1 from other respiratory tract infections. High proportions of pregnant women, bilateral respiratory consolidations, and presence of diarrhea and leukopenia in considerable number of patients were the most important findings of the study.