Background: Observing pneumothoraxes (PTXs) identified on chest computed tomography (CT) in mechanically ventilated patients remains highly debated. Despite the comorbidities associated with tube thoracotomy (TT), clinicians are inclined to perform this invasive procedure prophylactically. We hypothesize that PTX measuring ≤35 mm on chest CT can be safely observed in ventilated patients. Study design: A retrospective review was conducted of all patients diagnosed with PTX by chest CT between January 2011 and December 2016. Patients were excluded if they had an associated hemothorax (HTX), were not intubated, or had a TT placed before the initial chest CT. PTXs were measured as the radial distance between the parietal and the visceral pleura/mediastinum in a line perpendicular to the chest wall on axial imaging. Based on the previous work, a cutoff of 35 mm on the initial CT was used to dichotomize the groups. Failure of observation was defined as the need for a TT during the first week. A univariate analysis was performed to identify predictors of failure of observation in both groups. Results: A total of 116 patients met our inclusion criteria. Of those, 96 (83%) were successfully observed until discharge. Of those successfully observed, 88 (92%) patients had a measurement of ≤35 mm. In the univariate analyses, only the size of the PTX (≤35 mm or > 35 mm) (p = 0.001) was significantly associated with failing observation. The negative predictive value for 35 mm as a cutoff was 96.7% to predict successful observation. Conclusion: The 35 mm cutoff is safe as a general guide for ventilated patients with only 3% of stable patients failing initial observation.