Introduction: Anxiety is a common reaction in mechanically ventilated Intensive Care Unit (ICU) patients. However, due to their decreased physical condition and limited possibilities for communication, anxiety may be difficult to diagnose. The Short State Anxiety Inventory (SSAI) and the Faces Anxiety Scale (FAS) appear to be very useful for diagnosing anxiety in these patients. However, data are lacking on the use of these instruments in alert mechanically ventilated ICU patients in the Netherlands. The aim of this study was to compare sensitivity, specificity, reliability and practicality of the SSAI and FAS with the gold standard (The twenty-item State Anxiety Inventory) to evaluate their capacity for measuring anxiety. Materials and methods: A descriptive cross-sectional investigation of a random sample of 77 mechanically ventilated postoperative alert patients ventilated for ≤ 24 h (≤ 24 h) and 50 alert patients ventilated for > 24 h (> 24 h). Results: 43.8% of the postoperative ventilated patients and 68% of the patients ventilated for > 24 h experienced anxiety. In postoperative patients the SSAI had a sensitivity of 0.91 and a specificity of 0.90. The SSAI showed a sensitivity of 0.80 with a specificity of 0.91 in patients ventilated for > 24 h. The SSAI had an internal consistency of 0.75 and 0.81 for postoperative patients and 0.13 and 0.18 for patients ventilated for > 24 h (Cohen's kappa of 0.60 and 0.86, respectively). The response percentage was 98.2% (≤ 24 h) and 100% in patients ventilated (> 24 h). In the same groups, the FAS had a sensitivity of 0.48 (≤ 24 h) and 0.21 (> 24 h) with a specificity of 0.77 (≤ 24 h) and 0.48 (> 24 h); Cohen's kappa was 0.68 and 0.83 and the response percentage was 90% and 96%, in ≤ 24 h and > 24 h respectively. Conclusion: The SSAI appears to be more valid, reliable and easier to use than the FAS, and more appropriate for measuring anxiety in alert ventilated ICU patients in the Netherlands.