Objective: A large proportion of patients infected with 2009 influenza A(H1N1)(A[H1N1]) are obese. Obesity has been proposed as a risk factor influencing outcome in these patients. However, its role remains unclear. We evaluate the outcome of patients who are obese and infected with A(H1N1) in the ICU, determining whether obesity is a risk factor for mortality. Methods: This was a prospective, observational, and multicenter study performed in 144 ICUs in Spain. Data were obtained from the Grupo de Trabajo en Enfermedades Infecciosas de la Sociedad Española de Medicina Intensiva, Crítica y Unidades Coronarias (GTEI/SEMICYUC) registry. Adult patients with A (H1N1) that was confirmed by real-time polymerase chain reaction were included in the analysis. Patients who were obese (BMI>30) were compared with patients who were nonobese. Cox regression analysis was used to determine adjusted mortality. Differences of P<.05 were considered significant. Results: In January 2010, the GTEI/SEMICYUC registry had complete records for 416 patients. One hundred and fifty patients (36.1%) were obese, of whom 67 (44.7%) were morbidly obese (BMI>40). Mechanical ventilation (MV) was more frequently applied in patients who were obese (64% vs 52.4%, P<.01) Patients with obesity remained on MV longer than patients who were nonobese (6.5 ± 10.3 days vs 9.3 ± 9.7 days, P =.02), had longer ICU length of stay(10.8 ± 12.1 days vs 13.7 ± 11.7 days, P=.03), and had longer hospitalization (18.2 ± 14.6 days vs 22.2 ± 16.5 days, P=.02). Mortality adjusted by severity and potential confounders identified that obesity was not significantly associated with ICU mortality (hazard ratio, 1.1;95% CI, 0.69-1.75; P=.68). Conclusions: In our cohort, patients who were obese and infected with A (H1N1) did not have increased mortality. However, there was an association between obesity and higher ICU resource consumption. © 2011 American College of Chest Physicians.