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The characteristics of pandemic influenza 2009 differ from those of seasonal influenza. In Australia-New Zealand the number of admissions to the intensive care unit (ICU) increased by 15-fold in the southern winter. We compared the characteristics of the Spanish series of the first ICU admissions in July with those of series published in Canada and Australia-New Zealand up to October 2009. Unlike the situation in Spain, only half the admissions in Canada and Australia-New Zealand were due to primary viral pneumonia but bacterial pneumonia was much more frequent. In all series, young people, many of whom had no comborbidities, were the most frequently affected population. The most common comorbidities were obesity, chronic pulmonary disease, pregnancy and heart disease. Diagnosis through reverse-transcriptase polymerase chain reaction can have a false-negative rate of 10%. Shock and acute renal insufficiency were more frequent in the Spanish series. A total of 10-30% of patients required ICU admission and 6 of 10 patients required mechanical ventilation with a high frequency of failure of non-invasive ventilation (75%). Mortality was similar among the series (14-25%) but was higher in patients requiring mechanical ventilation (30%). Early oseltamivir administration (< 48h after symptom onset) has been associated with better outcome. Therefore, early administration of this drug in patients with risk factors or those who, although free from risk factors, show clinical progression, could reduce ICU admissions and mortality.