Background: Early mobilisation of mechanically ventilated patients during their stay at an intensive care unit (ICU) can improve physical recovery. Yet, an objective and specified description of physical activities while in the ICU is lacking. Therefore, our aim was to describe the objectively assessed type, quantity, and daily variation of physical activity among mechanically ventilated patients while in the ICU. Method: In an observational study in two mixed medical/surgical ICUs, we measured body posture in 39 patients on mechanical ventilation using a thigh- and chest-worn accelerometer while in the ICU. The accelerometer describes time spent lying, sitting, moving, in-bed cycling, standing and walking. Descriptive analysis of physical activity and daily variation was done using STATA. Results: We found that mechanically ventilated patients spend 20/24 h lying in bed, 3 h sitting and only 1 h standing, moving, walking or bicycling while in the ICU. Intervals of non-lying time appeared from 9.00 to 12.00 and again from 18.00 to 21.30, with peaks at the hours of 9.00 and 18.00. Conclusion: ICU patients on mechanical ventilation were primarily sedentary. Physical activity of mechanically ventilated patients seems to be related to nurse- and/or physiotherapy-initiated activities. There is a need to create an awareness of improving clinical routines, towards active mobilisation throughout the day, for this vulnerable patient population during their stay in the ICU.