Background: During the past decade, use has increased of moderate hypothermic circulatory arrest with antegrade cerebral perfusion for cerebral protection during aortic arch operations. This study examined the use of mild hypothermia in conjunction with unilateral selective antegrade cerebral perfusion (uSACP) for hemiarch replacement for proximal aortic arch reconstruction. Methods: A retrospective review of the Emory Aortic Database identified 708 patients who underwent aortic arch replacement between 2004 and 2011. Of these, 500 underwent hemiarch replacement at temperatures of 22°C or higher with uSACP. Outcomes were analyzed and compared between 277 patients undergoing hemiarch at a temperature of 28.6°C (mild) and 233 undergoing hemiarch at a temperature of 24.3°C (moderate). Propensity scores were generated and analyzed between the groups to adjust for confounding factors such as selection bias. Results: Operative mortality was equivalent between mild and moderate groups in elective (4.2% vs 4.8%, p = 0.80) and emergency (7.7% vs 11.7%, p = 0.43) settings. No differences occurred in the incidence of temporary neurologic dysfunction, dialysis-dependent renal failure, or mediastinal reexploration for bleeding between mild and moderate patients. The incidence of permanent neurologic deficit was significantly reduced in mild (2.5%) vs moderate patients (7.2%, p = 0.01), which was confirmed by the propensity score analysis (adjusted odds ratio, 0.28; p = 0.02). Conclusions: Hemiarch replacement can be safely performed at 28°C with uSACP in emergency and elective settings. Mild hypothermia with uSACP offers adequate levels of neurologic protection compared with deeper levels of hypothermia.