Abstract Background: Positive pressure ventilation has been shown to alter a variety of neurohormonal systems including sympa-thetic outflow, the renin-angiotensin axis, nonosmotic vaso-pressin (ADH) release and atrial natriuretic peptide (ANP) production. The end result of all of these neurohormonal pathways is diminished renal blood flow, decreased GFR, and fluid retention (salt and water) with oliguria. Aim of Study: To investigate the effect of positive pressure mechanical ventilator on aldosterone 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 meet 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 high PEEP (10-15) and group II with physiological PEEP (3-5).Aldosterone level was measured for these patients on first day of using PPV (aldosterone 1) then after 72h (aldosterone 2). Results: Aldosterone levels were increased with use mechanical ventilation and were significantly higher with higher level of PEEP. There were correlations between aldos-terone 2 with urea and Creatinine levels and with length of ICU stay only in patient group (p<0.001). Conclusion: Mechanical ventilation caused elevation of aldosterone levels. Aldosterone levels in mechanically venti-lated patients correlated with kidney function test (urea and Creatinine) and with length of ICU stay.