The purpose of this study was to establish the microbial aetiology and outcomes of patients with community-acquired pneumonia (CAP) treated as outpatients after presenting to a hospital emergency care unit. A prospective observational study was carried out in the Hospital Clinic of Barcelona (Barcelona, Spain). All consecutive cases of CAP treated as outpatients were included. 568 adult outpatients with CAP were studied (mean±SD age 47.2±17.6 yrs; 110 (19.4%) were aged o65 yrs). Aetiological diagnoses were established in 188 (33.1%) cases. Streptococcus pneumoniae was the most frequent pathogen followed by Mycoplasma pneumoniae and respiratory viruses. Legionella was detected in 13 (2.3%) cases. More than one causative agent was found in 17 (9.0%) patients. Mortality was low (three (0.5%) patients died) and other adverse events were rare (30 (5.2%) patients had complications, 13 (2.3%) were re-admitted and treatment failed in 13 (2.3%)). Complications were mostly related to pleural effusion and empyema, and readmissions and treatment failures to comorbidities. Outpatients with CAP have a characteristic microbial pattern. Regular antipneumococcal coverage remains mandatory. Treatment failures and re-admissions are rare and may be reduced by increased attention to patients requiring short-term observation in the emergency care unit and in the presence of pleural effusion and comorbidities. Copyright©ERS 2012.