OBJECTIVE: To determine the severity, duration, and contributing factors for metabolic acidosis after deep hypothermic circulatory arrest (DHCA). DESIGN: Retrospective observational study. SETTING: University hospital. PATIENTS: Eighty-seven consecutive patients undergoing elective thoracic aortic surgery with DHCA. INTERVENTIONS: Regression analysis was used to test for relationships between the severity of metabolic acidosis and clinical and laboratory variables. MEASUREMENTS AND MAIN RESULTS: Minimum pH averaged 7.27±0.06, with 76 (87%) having a pH<7.35; 55 (63%), a pH<7.30; and 7 (8%), a pH<7.20. The mean duration of metabolic acidosis was 7.9±5.0 hours (range: 0.0 - 26.8), and time to minimum pH after DHCA was 4.3±2.0 hours (1.0 - 10.0 hours). Hyperchloremia contributed to metabolic acidosis in 89% of patients. The severity of metabolic acidosis correlated with maximum lactate (p<0.0001) and hospital length of stay (LOS) (r = 0.22, p<0.05), but not with DHCA time, DHCA temperature, duration of vasoactive infusions, or ICU LOS. Patient BMI was the sole preoperative predictor of the severity of postoperative metabolic acidosis. LIMITATIONS: This retrospective analysis involved short-term clinical outcomes related to pH severity and duration, which indirectly may have included the impact of sodium bicarbonate administration. CONCLUSIONS: Metabolic acidosis was common and severe after DHCA and was attributed to both lactic and hyperchloremic acidosis. DHCA duration and temperature had little impact on the severity of metabolic acidosis. The severity of metabolic acidosis was best predicted by the BMI and had minimal effects on short-term outcomes. Preventing hyperchloremic acidosis has the potential to decrease the severity of metabolic acidosis after DHCA.