Obesity has been identified as a risk factor for adverse outcomes of 2009 H1N1 influenza. However, the impact of obesity on outcomes of infection remains controversial. There are limited data investigating the effect of obesity on outcomes of community-acquired pneumonia (CAP). This prospective observational study included patients presenting with CAP who had body mass index (BMI) measured on admission. Outcome measures included 30-day mortality and need for mechanical ventilation or inotropic support (MV/IS). 1079 patients were included, with 21%classified as obese (BMI =30 kg·m). Obesity was independently associated with reduced 30-day mortality from CAP on multivariate analysis (HR 0.53, 95%CI 0.29-0.98). This was not explained by differences in severity of disease on admission or requirement for MV/IS between obese and nonobese groups. Obese patients had higher median C-reactive protein levels and a higher frequency of sepsis using the systemic inflammatory response syndrome criteria (72.4%versus 64.1%; p50.03), than nonobese patients, suggesting greater systemic inflammation. Obesity was associated with reduced 30-day mortality in patients hospitalised with CAP.