This multicenter, prospective cohort study defined and validated a simple, clinically usable measure of clinical stability on hospital discharge for patients with community-acquired pneumonia. Unstable factors in the 24 hours prior to discharge were defined as temperature greater than 37.8°C (100°F), heart rate greater than 100/min, respiratory rate greater than 24/min, systolic blood pressure lower than 90 mm Hg, oxygen saturation lower than 90%, inability to maintain oral intake, and abnormal mental status. Among the 680 patients, 19.1% left the hospital with 1 or more instabilities on discharge. By 30 days, 10.5% of patients with no instabilities on discharge died or were readmitted compared with 13.7% of those with 1 instability and 46.2% of those with 2 or more instabilities (P<.003). Instability on discharge (defined as ≥1 unstable factor) was associated with higher risk-adjusted rates of death or readmission (odds ratio, l.6; 95% confidence interval, 1.0-2.8) and failure to return to usual activities (odds ratio, l.5; 95% confidence interval, 1.0-2.4). Patients with 2 or more unstable factors on discharge had dramatically increased risk-adjusted rates of death or readmission (odds ratio, 5.4; 95% confidence interval, 1.6-18.4).