WHAT IS ALREADY KNOWN ON THIS TOPIC: Procalcitonin is a biomarker that appears to correlate with bacterial infection. WHAT QUESTION THIS STUDY ADDRESSED: Does a procalcitonin level add prognostic information for pneumonia patients in conjunction with scoring systems such as the Pneumonia Severity Index or CURB-65? WHAT THIS STUDY ADDS TO OUR KNOWLEDGE: Among 1,651 patients with community-acquired pneumonia in 28 US emergency departments, procalcitonin levels did not add prognostic information for most pneumonia patients. Among higher-risk groups by Pneumonia Severity Index score, low procalcitonin level predicted lower mortality. HOW THIS MIGHT CHANGE CLINICAL PRACTICE: Clinicians should continue using validated prognostic scoring systems for pneumonia. Low procalcitonin level could be considered as a factor for selected patients who would otherwise be considered high risk to be treated in a lower acuity setting.