Background: EAST guidelines suggest tube thoracostomy (TT) be considered for all traumatic hemothoraces. However, previous research has suggested that some traumatic hemothoraces may be safely observed. We sought to 1) determine the safety of selective observation for traumatic hemothorax, and 2) identify predictors of failed observation. Methods: All patients with traumatic hemothorax from 2000-2014 at a Level I Trauma Center were identified and categorized by size as small (<300cc) or large (≥300cc) based on chest CT-scan measurements. Patients with no CT or with TT placement prior to CT were excluded. Patients were also categorized into four intervention groups: i) early TT (<24hr after CT), ii) failed observation (TT ≥24hr after CT, including during readmissions), iii) successful observation (no TT), and iv) inevaluable due to early mortality (no TT but died within 7 days). Univariate analyses compared outcomes between the three groups. Multivariate analyses identified independent predictors of failed observation. Results: 340 patients met inclusion criteria. 156 (46%) hemothoraces were managed with early TT. Of the 184 patients that were initially observed, 121 (66%) were successfully observed, 53 (29%) failed observation, and 10 (5%) were inevaluable due to early mortality. Almost all of the successfully observed hemothoraces were small (119/121, 98%). Four independent predictors of failed observation were identified: older age, fewer ventilation-free days, large hemothorax, concurrent pneumothorax. Patients who received TT were more likely than non-TT patients to receive tPA, develop an empyema, have fewer hospital-free days, and be discharged to rehabilitation rather than home. When compared to early TT, failed observation was associated with a higher likelihood of discharge to rehabilitation but no difference in mortality, hospital-free days, or rate of empyema. Conclusions: Initial observation in select patients is safe and may result in better outcomes. The identified predictors of failed observation can help clinical decision-making regarding the need for TT in patients with traumatic hemothorax.Scholarly Project