From 1 July 2009 to 15 November 2009, 244 patients with 2009 pandemic influenza A(H1N1) were admitted to intensive care units (ICU) and were compared with 514 cases hospitalised in medical wards in France until 2 November 2009. Detailed case-based epidemiological information and outcomes were gathered for all hospitalised cases. Infants and pregnant women are overrepresented among cases admitted to ICU with seven per cent for both groups respectively, and twenty per cent of ICU cases did not belong to a risk group. Chronic respiratory disease was the most common risk factor among cases but obesity (body mass index = 30 Kg/m2), chronic cardiac disease and immunosuppression were risk factors associated with severe illness after adjustment for age and for other co-morbidities.