BACKGROUND: There are limited data describing how pre-existing heart failure affects mortality following pneumonia. OBJECTIVE: To examine the association between history and severity of heart failure and mortality among patients hospitalized for pneumonia. DESIGN: Population-based cohort study in Western Denmark between 1994 and 2003. PATIENTS: 33,736 adults with a first-time hospitalization for pneumonia. Heart failure was identified and categorized based on data linked from population-based health care databases. MEASUREMENTS: We compared 30-day mortality between patients with pre-existing heart failure and other pneumonia patients, while adjusting for age, gender, comorbidity, and medication use. RESULTS: The 30-day mortality was 24.4% among heart-failure patients and 14.4% among other patients, with an adjusted 30-day mortality rate ratio (MRR) of 1.40 (95% CI: 1.29-1.51). Adjusted MRRs increased according to severity of pre-existing heart failure, as indicated by medication regimen: thiazide-based, MRR=1.09 (95% CI: 0.79-1.50); loop-diuretics, MRR=1.25 (95% CI: 1.10-1.43); loop-diuretics and digoxin, MRR=1.35 (95% CI: 1.18-1.55); loop-diuretics and spironolactone, MRR=1.72 (95% CI: 1.49-2.00). Pre-existing heart valve disease and atrial fibrillation substantially increased mortality. CONCLUSION: History and severity of heart failure are associated with a poor outcome for patients hospitalized with pneumonia. © 2008 Society of General Internal Medicine.