OBJECTIVE: To compare ideal PaO2 with PaO2 found, ideal PaO2/FiO2 of room air with the one found, and ideal FiO2 with FiO2 found in mechanically ventilated elderly patients. METHODS: Cross-sectional study that evaluated elderly mechanically ventilated patients for at least 72 hours and who underwent three subsequent blood gas analyses. RESULTS: The sample consisted of 48 elderly with mean age of 74.77±9.36 years. There was a significant difference between the ideal PaO2 and the one found (p