Background: Acute respiratory failure in chronic respiratory disease remains a major reason for mechanical ventilation in ICU patients. Aim of study: to identify prevalence and risk factors of acute respiratory failure among critically ill patients at Assiut University Hospital. Subject and Method: All patients diagnosed with acute Respiratory failure within 6 months period admitted in to chest intensive care unit and trauma at Assiut University Hospital. Tools of data collection: Tool (1) Acute respiratory failure patient assessment sheet, Tool (2) outcomes of respiratory failure. Results: Risk factors of acute respiratory failure among critically ill patients in the above mentioned setting at Assuit University Hospital was 150 patients who exposed to respiratory failure. About that90.8%in the chest ICU groups were smoking and 76.7% were suffer from COPD but Injury of spine, brain or chest and smoking were the majority risk factors in the trauma ICU groups (50% and 43,3% respectively). With statistical significant differences between both groups (p=0.000*).Conclusion: There was highly statistical significant differences relationship between chest ICU groups and trauma ICU groups in smoking, COPD, occupation and the prevalence of Acute respiratory failure with (p=0.000*). Recommendations: Application of life style modification for modifying risk factors leading to ARF