Distinguishing between bacterial and H1N1 infection in patients with acute respiratory distress syndrome (ARDS) is difficult based on clinical symptoms alone; moreover, rapid antigen test for influenza has poor sensitivity. The aim of this work was to determine whether the differential cell count in bronchoalveolar lavage (BAL) fluid could aid in early diagnosis of H1N1-ARDS. We retrospectively identified pneumonia-induced ARDS patients who underwent BAL in intensive care unit (ICU) of the Chang Gung Memorial Hospital, Keelung from January 1, 2014 to March 31, 2016. Patient characteristics, severity of illness scores, white blood count (WBC) and differential count, biochemical test, BAL fluid differential cell count, and semi-quantitative culture of lower respiratory tract sample were evaluated. Nine patients with H1N1-ARDS and 18 with non-H1N1-ARDS were identified. Patients with H1N1-ARDS had lower APACHE II scores. Lymphocyte percentage in BAL fluid was significantly higher in the H1N1-ARDS group (15.6 ± 7.5% vs. 7.6 ± 8.0%, p=0.009). The area under the ROC curve (AUC) was 0.829; with a sensitivity of 85.5%, specificity of 77.8%, positive predictive value of 79.4% and negative predictive value of 84.3% for H1N1-ARDS prediction at a cutoff value of 11%. Lymphocyte percentage in BAL fluid was higher in patients with H1N1-ARDS than in those without. This result has potential applicability for early detection of H1N1 influenza virus infection in patients with ARDS.