Background - The emergence of novel A(H1N1)pdm2009 virus threatened to lead to frequent severe manifestations. Objectives - To describe the clinical, virological, and biological characteristics of the disease and identify the factors associated with severe presentations. Study design - This prospective multicenter study recruited consecutive hospitalized patients with confirmed A(H1N1)pdm2009 disease. Clinical, virological and biological assessments were carried out at inclusion and 30 days post-inclusion. Disease manifestations were assessed by an adjudication committee using pre-identified definitions of complications and severity scores. Results - The study analyzed from November 30th, 2009 to February 8th, 2010, 40 hospitalized patients, 21 children and 19 adults. Eighteen (45%) were considered to have severe presentations. Except age, main characteristics in children and adults did not differ. The majority (18/21) of children and all adults had a respiratory presentation; extra-respiratory manifestations tended to be more frequent in children (12 vs. 6, P = 0.10). Two children against 5 adults presented acute respiratory distress syndrome (ARDS, P = 0.23), but more children suffered respiratory failure (7 vs. 1, P = 0.046) without ARDS. At day 30, one death had occurred in each group. The main factor associated with non-severe presentation was an early (