Abstract Background: Mechanical ventilation can be lifesaving but it is also associated with numerous complications. The incidence of some complications increases with duration of mechanical ventilation, its effects on renal function results in decreas GFR, and fluid retention. Aim of Study: To investigate the effect of positive pressure mechanical ventilator on renin and to study its prognostic value. Patients and Methods: This study was conducted on 50 patients admitted to the intensive care unit (ICU) and needed mechanical ventilation and did not met any of exclusion criteria and 25 control (critically ill patients who did not need mechanical ventilator). The patients were divided into 2 equal groups; group I with use high PEEP (10-15) and group II with physiological PEEP (3-5). Renin levels were measured for these patients on first day of using PPV (renin 1) then after 72h (renin 2). Results: Renin levels were increased with use mechanical ventilation and were significantly higher with higher level of PEEP. There was correlation between renin with urea and Creatinine levels and significant correlation with APACH IV score and length of mechanical ventilation only in patients. Renin levels were significant higher in patients need hemo-dynamic support and in non survivors. We estimated an optimal cutoff values for renin 1 and renin 2 level of (41, 51µg/L respectively) to predict ICU mortality with moderate sensitivity and specificity. Conclusion: Mechanical ventilation caused elevation of renin levels especially with higher levels of PEEP. Renin levels in mechanically ventilated patients correlated with kidney function test (urea and Creatinine) and with length of mechanical ventilation and with APACH IV score. Values for renin 1 and renin 2 of (41, 5 1 m g/L respectively) could be used as a cutoff value to predict ICU mortality with moderate sensitivity and specificity.