Background— More than 300 000 out-of-hospital cardiac arrests (OHCA) occur each year in the United States. The relationship between time of day and OHCA outcomes in the prehospital setting is unknown. Any such association may have important implications for emergency medical services resource allocation. Methods and Results— We performed a retrospective review of cardiac arrest data from a large, urban emergency medical services system. Included were OHCA occurring in adults from January 2008 to February 2012. Excluded were traumatic arrests and cases in which resuscitation measures were not performed. Day was defined as 8 am to 7:59 pm ; night, as 8 pm to 7:59 am . A relative risk regression model was used to evaluate the association between time of day and prehospital return of spontaneous circulation and 30-day survival, with adjustment for clinically relevant predictors of survival. Among the 4789 included cases, 1962 (41.0%) occurred at night. Mean age was 63.8 years (SD, 17.4 years); 54.5% were male. Patients with an OHCA occurring at night did not have significantly lower rates of prehospital return of spontaneous circulation compared with patients having daytime arrests (11.6% versus 12.8%; P =0.20). However, rates of 30-day survival were significantly lower at night (8.56% versus 10.9%; P =0.02). After adjustment for demographics, presenting rhythm, field termination, duration of call, dispatch-to-scene interval, automated external defibrillator application, bystander cardiopulmonary resuscitation, and location, 30-day survival remained significantly higher after daytime OHCA, with a relative risk of 1.10 (95% confidence interval, 1.02–1.18). Conclusion— Rates of 30-day survival were significantly higher for OHCA occurring during the day compared with at night, even after adjustment for patient, event, and prehospital care differences.