The aim of this study was to assess the association between obesity and the risk of intensive care unit (ICU) admission and death among patients hospitalized for influenza A (H1N1) viral infection. A systematic review of the Medline and Cochrane databases using 'obesity', 'hospitalization', 'influenza A viral infection', various synonyms, and reference lists of retrieved articles from January 2009 to January 2010. Studies comparing the prevalence of obesity among patients with confirmed infection for influenza A virus and who were either hospitalized or admitted to ICU/died were included. A total of 3059 subjects from six cross-sectional studies, who were hospitalized for influenza A (H1N1) viral infection, were included in this meta-analysis. Severely obese H1N1 patients (body mass index >= 40 kg m(-2), n = 804) were as twice as likely to be admitted to ICU or die (odds ration: 2.01, 95% confidence interval: 1.29-3.14, P < 0.002) compared with H1N1 patients who were not severely obese. Having a body mass index >= 30 kg m(-2) was similarly associated with a more than twofold increased risk of ICU admission or death although this did not reach statistical significance (2.14, 0.92-4.99, P < 0.07). This meta-analysis supports the view that obesity is associated with higher risks of ICU admission or death in patients with influenza A (H1N1) infection. Therefore, morbid obese patients should be monitored more intensively when hospitalized.