Background: Critically ill patients receiving prolonged mechanical ventilatory support are at risk for loss of muscle mass and muscle strength from immobility. Feasible, non-invasive methods are needed to accurately obtain data on markers of muscle mass to design effective interventions and monitor patient progress during recovery from critical illness. Bioelectric impedance has been used in other settings to obtain data on body composition and muscle mass. Purpose: The aims of this study were to determine the feasibility of bioelectric impedance as a marker of muscle mass in a sample of mechanically ventilated patients and to assess data trends in these obtained values. Methods: A descriptive design was used to obtain standard bioelectric impedance parameters (total body resistance, legs resistance, and percent lean body mass) over 4 days from eligible patients already enrolled in a randomized clinical trial. Results: Bioimpedance parameters were readily obtained over 4 days in a sample of 43 patients (age 59 + 15.7 years, 56% male) receiving prolonged ventilatory support (mean 9.4 + 10.4 days) due to respiratory failure. Reasons for not obtaining impedance measures included skin impairment, monitoring devices, or presence of implantable cardiac defibrillator or pacemaker. Average total body impedance was 464.3 + 117.1 ohms, while average impedance of legs was 479.1 + 146.4 ohms. Lean body mass was 68.4% (+10.8). Conclusions/Implications for Practice: With carefully trained staff and a standardized measurement protocol, bioimpedance is a feasible method to obtain body composition data reflective of muscle mass in mechanically ventilated patients. Further research will determine the utility of bioimpedance to monitor recovery and effectiveness of interventions to restore function after prolonged periods of ventilatory support and immobility in mechanically ventilated patients.