BACKGROUND: Although the Pneumonia Severity Index (PSI) has been extensively validated, little is known of the impact of its routine use as an aid to site-of-treatment decisions for patients with pneumonia who present to emergency departments (EDs). METHODS: A prospective, observational, controlled cohort study of patients with pneumonia was conducted in 8 EDs that used the PSI (PSI-user EDs) and 8 EDs that did not use the PSI (PSI-nonuser EDs) in France. The outcomes examined included the proportion of "low-risk" patients (PSI risk classes I-III) treated as outpatients, all-cause 28-day mortality, admission of inpatients to the intensive care unit, and subsequent hospitalization of outpatients. RESULTS: Of the 925 patients enrolled in the study, 472 (51.0%) were treated at PSI-user EDs, and 453 (49.0%) were treated at PSI-nonuser EDs; 449 (48.5%) of all patients were considered to be at low risk. In PSI-user EDs, 92 (42.8%) of 215 patients at low risk were treated as outpatients, compared with 56 (23.9%) of 234 patients at low risk in PSI-nonuser EDs. The adjusted odds ratios for outpatient treatment were higher for patients in PSI risk classes I and II who were treated in PSI-user EDs, compared with PSI-nonuser EDs (adjusted odds ratio, 7.0 [95% confidence interval, 2.0-25.0] and 4.6 [95% confidence interval, 1.3-16.2], respectively), whereas the adjusted odds ratio did not differ by PSI-user status among patients in risk class III or among patients at high risk. After adjusting for pneumonia severity, mortality was lower in patients who were treated in PSI-user EDs; other safety outcomes did not differ between patients treated in PSI-user and PSI-nonuser EDs. CONCLUSIONS: The routine use of the PSI was associated with a larger proportion of patients in PSI risk classes I and II who had pneumonia and who were treated in the outpatient environment without compromising their safety.