Background— Community-acquired pneumonia (CAP) affects >5 million adults each year in the United States. Although incident cardiac complications occur in patients with community-acquired pneumonia, their incidence, timing, risk factors, and associations with short-term mortality are not well understood. Methods and Results— A total of 1343 inpatients and 944 outpatients with community-acquired pneumonia were followed up prospectively for 30 days after presentation. Incident cardiac complications (new or worsening heart failure, new or worsening arrhythmias, or myocardial infarction) were diagnosed in 358 inpatients (26.7%) and 20 outpatients (2.1%). Although most events (89.1% in inpatients, 75% in outpatients) were diagnosed within the first week, more than half of them were recognized in the first 24 hours. Factors associated with their diagnosis included older age (odds ratio [OR]=1.03; 95% confidence interval [CI], 1.02–1.04), nursing home residence (OR, 1.8; 95% CI, 1.2–2.9), history of heart failure (OR, 4.3; 95% CI, 3.0–6.3), prior cardiac arrhythmias (OR, 1.8; 95% CI, 1.2–2.7), previously diagnosed coronary artery disease (OR, 1.5; 95% CI, 1.04–2.0), arterial hypertension (OR, 1.5; 95% CI, 1.1–2.1), respiratory rate ≥30 breaths per minute (OR, 1.6; 95% CI, 1.1–2.3), blood pH <7.35 (OR, 3.2; 95% CI, 1.8–5.7), blood urea nitrogen ≥30 mg/dL (OR, 1.5; 95% CI, 1.1–2.2), serum sodium <130 mmol/L (OR, 1.8; 95% CI, 1.02–3.1), hematocrit <30% (OR, 2.0; 95% CI, 1.3–3.2), pleural effusion on presenting chest x-ray (OR, 1.6; 95% CI, 1.1–2.4), and inpatient care (OR, 4.8; 95% CI, 2.8–8.3). Incident cardiac complications were associated with increased risk of death at 30 days after adjustment for baseline Pneumonia Severity Index score (OR, 1.6; 95% CI, 1.04–2.5). Conclusions— Incident cardiac complications are common in patients with community-acquired pneumonia and are associated with increased short-term mortality. Older age, nursing home residence, preexisting cardiovascular disease, and pneumonia severity are associated with their occurrence. Further studies are required to test risk stratification and prevention and treatment strategies for cardiac complications in this population.