OBJECTIVE: The relationship between environmental factors, such as winter or cold environments, and the onset of out-of-hospital cardiac arrest (OHCA) is well known. However, the association between environmental factors and the neurologic outcome of OHCA is poorly understood. This study aimed to assess the impact of the ambient temperature on the neurologic outcome of adult OHCA. METHODS: In a nationwide, population-based, observational study, we enrolled 121,081 adults 18 years or older who experienced an OHCA from January 1, 2010, to December 31, 2010. We used the All-Japan Utstein Registry database coupled with climate statistics data from the Japan Meteorological Agency. The primary end point was favorable neurologic outcome 1 month after OHCA. RESULTS: Of the eligible 120,721 adult patients with OHCA, 7747 cases of OHCA (6.4%) occurred during the cold season, 80,739 (66.9%) occurred during the midseason, and 32,235 (26.7%) occurred during the warm season. The adults who experienced an OHCA during the cold season exhibited a significantly lower rate of a favorable neurologic outcome than did those who experienced an OHCA during the warm season (2.4% vs 3.3%; odds ratio, 0.73; 95% confidence interval, 0.62-0.85; P < .0001). The adjusted odds ratio for favorable neurologic outcome per 1 degrees C increase in the monthly ambient temperature was 1.006 (95% confidence interval, 1.002-1.010; P = .0080). CONCLUSIONS: The seasonal ambient temperature is likely to affect favorable neurologic outcome. A lower seasonal ambient temperature may exacerbate the neurologic outcome of OHCA.