Background: The pandemic influenza A (H1N1) 2009 was a global outbreak of a new strain of influenza A virus. The associated factors of influenza A (H1N1) 2009 infection in Thailand have not been well studied.
Objective: To determine the associated factors for diagnosis of influenza A (H1N1) 2009. 
Study design: Retrospective analytical study.
Material and Method: A total 430 patients who had influenza-like symptoms from the out-patient department of Rajavithi Hospital in Thailand were analyzed by collecting the hospital charts of the patients between 1 June 2009 and 31 August 2009, total 3 months. The patients’ personal information, signs and symptoms, laboratory data and throat swab for influenza A (H1N1) 2009 virus confirmed by real time reverse transcription polymerase chain reaction were analyzed.
Results: There were 152 (35.3%) influenza A (H1N1) 2009 confirmed-case patients, 100 (65.8%) were female and 52 (34.2%) were male. The average age was 28.2 + 15.6 years and the most occupation were housewives (56.6%) and students (36.8%). The patients presented with acute fever (100.0%), cough (95.4%), rhinorrhea (66.4%), dyspnea (27.0%) and the mean duration of fever was 4.3 + 2.9 days at the time of sample collection. Laboratory finding such as leukocytes count, platelets count, lactate dehydrogenase and chest x-ray were normal in most cases. The associated factors of influenza A (H1N1) 2009 infection were age < 35, pregnancy, rhinorrhea and white blood count < 10,000 cells/ml3.
Conclusion: The associated factors of influenza A (H1N1) 2009 were age < 35 years, pregnancy, rhinorrhea and white blood count < 10,000 cells/ml3.

Keywords: Influenza A (H1N1) 2009, Associated factor