RATIONALE: Pneumonia due to Pseudomonas aeruginosa (PA) is associated with high mortality and requires antipseudomonal treatment. Since PA can colonize the respiratory tract, the diagnosis of pathogenic PA involvement is challenging. OBJECTIVES: To determine the prevalence of definitive and indeterminate PA infection in community-acquired pneumonia, to describe the clinical and microbiological profiles and to estimate the burden of unnecessary antipseudomonal drug prescriptions. METHODS: We prospectively enrolled 2,701 patients with community-acquired pneumonia. Using stringent criteria for diagnosing PA pneumonia, we generated three groups: 1) definitive PA, 2) indeterminate PA, and 3) non-PA pneumonia. RESULTS: The prevalence of definitive PA pneumonia was 0.9% (n = 25) and that of indeterminate PA pneumonia was 4.9% (n = 131). Considerable clinical differences were observed among the groups. Patients with definitive PA pneumonia were more likely to have a history of tuberculosis and COPD/bronchiectasis and had a higher 30-day mortality (28%) than patients with non-PA pneumonia. Patients with indeterminate PA pneumonia were more likely to have comorbidities than patients with non-PA pneumonia. More than half of the patients with indeterminate PA and 25% of the patients with non-PA pneumonia were treated with an antipseudomonal drug. No patients with definitive PA pneumonia had multidrug resistance. CONCLUSIONS: In this population, the prevalence of community-acquired pneumonia due to PA was low. The clinical features and 30-day mortality rates of indeterminate PA pneumonia were different from those of definitive PA pneumonia. Most of the prescribed antipseudomonal drugs for patients with community-acquired pneumonia were potentially unnecessary.