Obesity is an increasing problem in the USA, and research into the association between obesity and pneumonia has yielded conflicting results. Using Department of Veterans Affairs administrative data from fiscal years 2002-2006, we examined a cohort of patients hospitalised with a discharge diagnosis of pneumonia. Body mass index was categorised as underweight (<18.5 kg · m(-2)), normal (18.5-24.9 kg · m(-2), reference group), overweight (25-29.9 kg · m(-2)), obese (30-39.9 kg · m(-2)) and morbidly obese (≥ 40 kg · m(-2)). Our primary analyses were multi level regression models with the outcomes of 90-day mortality, intensive care unit (ICU) admission, need for mechanical ventilation and vasopressor utilisation. The cohort comprised 18 746 subjects: 3% were underweight, 30% were normal, 36% were overweight, 27% were obese and 4% were morbidly obese. In the regression models, after adjusting for potential confounders, morbid obesity was not associated with mortality (OR 0.96, 95% CI 0.72-1.28), but obesity was associated with decreased mortality (OR 0.86, 95% CI 0.74-0.99). Neither obesity nor morbid obesity was associated with ICU admission, use of mechanical ventilation or vasopressor utilisation. Underweight patients had increased 90-day mortality (OR 1.40, 95% CI 1.14-1.73). Although obesity is a growing health epidemic, it appears to have little impact on clinical outcomes and may reduce mortality for veterans hospitalised with pneumonia.