SUMMARY. Background. Monitoring of respiratory mechanics during mechanical ventilation usually focuses on elastance (Ers) and resistance (Rrs). Evaluation of the impedance (Zrs), reactance (Xrs) and phase angle (θrs) seem sophisticated in clinical intensive care unit (ICU) practice. This study examines the diagnostic utility of a detailed mechanical profile composed with the aid of Fourier Analysis (FA). Patients and methods. Thirty mechanically ventilated patients (CMV mode) were classified into 3 groups: a) without respiratory disease (WRD) (n=10), b) with chronic obstructive pulmonary disease (COPD) exacerbation (n=9) and c) with acute respiratory distress syndrome (ARDS) (n=11). FA produced values of Zrs and θrs, from which Rrs, Xrs and Ers were derived. Results. Ers was significantly higher and Xrs significantly lower in ARDS. Rrs and Zrs were significantly lower in WRD while θrs was significantly higher (less negative) in COPD. The detailed mechanical profile is characteristic for each group. Conclusions. The primarily calculated parameters, Zrs and θrs, examined concomitantly may strongly indicate the predominant mechanical disorder and help in the differential diagnosis between the three entities, WRD, COPD and ARDS. Pneumon 2007; 20(2):187 193.