Background: Acute Respiratory Failure (ARF) is an important presentation in emergency department (ED). Intubation and mechanical ventilation (MV) are sometimes the ultimate decisions in such emergent situations. Many of these patients are unfortunately managed in an ED. This may endanger their outcome. This study was aimed to compare the outcome of critically ill patients with ARF under MV admitted to ED with patients admitted to ICU. Methods: All critically ill patients with ARF who were intubated and placed under MV were enrolled in this study. Cases were either admitted to ED or ICU. Demographic data, initial diagnosis, final diagnosis, length of hospital stay, one-month mortality rate, number of times patient was intubated and number of times patient was resuscitated were all recorded and compared between the 2 groups. Results: Out of all 172 patients admitted to ED and ICU, 74 (43%) were females and 98 (57%) were males. Patients had a mean±, SD age of 64. 2±, 18. 1 years. ICU patients (85 patients (49. 4%)) had a significantly longer hospital stay (p=0. 048). ICU patients had significantly more resuscitation process (p=0. 006). Mortality rate in ICU was significantly higher than ED. In the meanwhile, more patients in ED finally survived (p=0. 004). Conclusion: In the present study, ICU patients were admitted longer to the ward than ED patients. Overall, ED mortality rate was lower than ICU. More patients finally survived in ED in comparison to ICU.