Background: Since the start of the 2009 influenza A pandemic (H1N1pdm), the World Health Organization and its memberstates have gathered information to characterize the clinical severity of H1N1pdm infection and to assist policy makers todetermine risk groups for targeted control measures.Methods and Findings: Data were collected on approximately 70,000 laboratory-confirmed hospitalized H1N1pdmpatients, 9,700 patients admitted to intensive care units (ICUs), and 2,500 deaths reported between 1 April 2009 and 1January 2010 from 19 countries or administrative regions—Argentina, Australia, Canada, Chile, China, France, Germany,Hong Kong SAR, Japan, Madagascar, Mexico, the Netherlands, New Zealand, Singapore, South Africa, Spain, Thailand, theUnited States, and the United Kingdom—to characterize and compare the distribution of risk factors among H1N1pdmpatients at three levels of severity: hospitalizations, ICU admissions, and deaths. The median age of patients increased withseverity of disease. The highest per capita risk of hospitalization was among patients ,5 y and 5–14 y (relative risk [RR] = 3.3and 3.2, respectively, compared to the general population), whereas the highest risk of death per capita was in the agegroups 50–64 y and $65 y (RR = 1.5 and 1.6, respectively, compared to the general population). Similarly, the ratio ofH1N1pdm deaths to hospitalizations increased with age and was the highest in the $65-y-old age group, indicating thatwhile infection rates have been observed to be very low in the oldest age group, risk of death in those over the age of 64 ywho became infected was higher than in younger groups. The proportion of H1N1pdm patients with one or more reportedchronic conditions increased with severity (median = 31.1%, 52.3%, and 61.8% of hospitalized, ICU-admitted, and fatalH1N1pdm cases, respectively). With the exception of the risk factors asthma, pregnancy, and obesity, the proportion ofpatients with each risk factor increased with severity level. For all levels of severity, pregnant women in their third trimesterconsistently accounted for the majority of the total of pregnant women. Our findings suggest that morbid obesity might bea risk factor for ICU admission and fatal outcome (RR = 36.3).Conclusions: Our results demonstrate that risk factors for severe H1N1pdm infection are similar to those for seasonalinfluenza, with some notable differences, such as younger age groups and obesity, and reinforce the need to identify andprotect groups at highest risk of severe outcomes.