From 25th April 2009 to 20th January 2010, 771 laboratory confirmed cases of 2009 pandemic influenza A (H1N1) were admitted to intensive care units (ICU) or hospitalized in medical wards with clinical criteria of severe disease (hospitalized pneumonias, multiorganic failure, septic shock or admitted to ICU or death while hospitalized). 82% of cases were hospitalized between epidemiological week 43 and 48 (25th October – 5th December 2009). Median age of patients was 40 years (range 0-89 years) and 56% were males. 38.7% cases were in the 15-44 year age-group, 29.4% in the 45-64 and 21.8% were children under 15 years of age. 36.8% were admitted to an ICU and 48 died. Underlying conditions were absent in 29% of patients (up to 38% among those under 15 years old). Differences in the prevalence of underlying conditions were found between children and adults. In children less than 15 years old, asthma (16.2%), other respiratory diseases (12.7%), cognitive disorders (10.2%), epilepsy (8.7%) and neuromuscular disorders (7.1%) were the most frequent. In adults, chronic obstructive pulmonary disease (14.8%), cardiovascular diseases (12.6%), asthma (11.5%), diabetes (11.2%) and morbid obesity (10.6%) were the most frequent. Further surveillance is needed to better characterize the epidemiology of this pandemic.