We describe characteristics of patients admitted to our intensive care unit with severe acute respiratory illness and influenza-like syndrome during the first months of the pandemic influenza A(H1N1) 2009 in Argentina. We analyzed clinical data, severity scores, laboratory tests, microbiological and radiological findings at admission, clinical outcomes and in-hospital mortality. H1N1 was confirmed by RT-PCR. Data from positive and negative PCR patients were compared. We admitted 31 adult patients between June and July 2009; median age: 54 years (IQR 33-66). A 54% (17) had positive PCR; 16 patients presented underlying medical conditions. Bilateral interstitial opacities were observed in chest radiography in 20 cases; 5 had unilateral lobar consolidation. Bacterial co-infection (isolation or IgM antibodies for bacterial infections) was found in 21 patients. Mechanical ventilation was required in 23 patients and 18 developed ARDS. Lymphopenia and increased creatine kinase levels were frequently observed (83% and 65% among PCR+ and PCR- respectively). Six patients died (19%); they were all over 75 years old, had cancer or immune-suppression. Early antiviral treatment (≤ 48 hours from symptoms onset) was associated with less frequency of mechanical ventilation (54% vs. 89%, p: 0.043). There were no differences in analyzed variables when comparing H1N1 positive and H1N1 negative patients; which suggests this approach as a most correct in future epidemic outbreaks. H1N1 infection was associated with severe respiratory illness and ARDS. Fatal outcome was observed in very old patients, or in those with major co-morbidities.