INTRODUCTION: Our objectives were to determine the causes of acute respiratory failure (ARF) in elderly patients and to assess the accuracy of the initial diagnosis by the emergency physician, and that of the prognosis. METHOD: In this prospective observational study, patients were included if they were admitted to our emergency department, aged 65 years or more with dyspnea, and fulfilled at least one of the following criteria of ARF: respiratory rate at least 25 minute-1; arterial partial pressure of oxygen (PaO2) 70 mmHg or less, or peripheral oxygen saturation 92% or less in breathing room air; arterial partial pressure of CO2 (PaCO2) > or = 45 mmHg, with pH < or = 7.35. The final diagnoses were determined by an expert panel from the completed medical chart. RESULTS: A total of 514 patients (aged (mean +/- standard deviation) 80 +/- 9 years) were included. The main causes of ARF were cardiogenic pulmonary edema (43%), community-acquired pneumonia (35%), acute exacerbation of chronic respiratory disease (32%), pulmonary embolism (18%), and acute asthma (3%); 47% had more than two diagnoses. In-hospital mortality was 16%. A missed diagnosis in the emergency department was noted in 101 (20%) patients. The accuracy of the diagnosis of the emergency physician ranged from 0.76 for cardiogenic pulmonary edema to 0.96 for asthma. An inappropriate treatment occurred in 162 (32%) patients, and lead to a higher mortality (25% versus 11%; p < 0.001). In a multivariate analysis, inappropriate initial treatment (odds ratio 2.83, p < 0.002), hypercapnia > 45 mmHg (odds ratio 2.79, p < 0.004), clearance of creatinine < 50 ml minute-1 (odds ratio 2.37, p < 0.013), elevated NT-pro-B-type natriuretic peptide or B-type natriuretic peptide (odds ratio 2.06, p < 0.046), and clinical signs of acute ventilatory failure (odds ratio 1.98, p < 0.047) were predictive of death. CONCLUSION: Inappropriate initial treatment in the emergency room was associated with increased mortality in elderly patients with ARF.