Objective: To evaluate pandemic influenza A (H1N1) 2009 in hospitalized patients in order to identify risk factors for hospitalization and, consequently, for the worsening of the disease. Methods: This retrospective observational study was conducted between March and December of 2010. The data were collected from the Brazilian Ministry of Health National Case Registry Database. We included only patients (inpatients and outpatients) in whom H1N1 infection was confirmed (via laboratory testing) during the study period. The variables regarding demographic and clinical characteristics were statistically evaluated in order to compare the hospitalization rates in the presence or absence of these factors. Risk factors were identified by logistic regression analysis. Results: We included 4,740 patients with laboratory confirmation of H1N1 infection. Of these, 1,911 individuals were hospitalized, and 258 (13.5%) died. The risk factors for hospitalization were age (20-29 years), African or Indigenous ethnicity, presence of specific comorbidities (heart disease, lung disease, kidney disease, hemoglobinopathy, immunosuppression, diabetes, obesity, puerperium, and smoking), a high number of comorbidities, and specific symptoms (dyspnea, diarrhea, vomiting, chest pain, hemoptysis, pneumonia, and wheezing). Higher levels of education and early use of oseltamivir were found to be protective factors. Hospitalization contributed to an increase in survival. Conclusions: Knowledge of the epidemiological characteristics that can be associated with hospitalization, disease severity, and mortality can be helpful in the adoption of preventive measures, as well as in the early diagnosis and treatment of disease, which might contribute to the reduction in the numbers of hospitalizations and deaths.

Keywords: Influenza A virus, H1N1 subtype; Hospitalization; Risk factors; Oseltamivir.