© 2018 American College of Cardiology Foundation Background: Survival after in-hospital cardiac arrest (IHCA) is lower during nights and weekends (off-hours) compared with daytime during weekdays (on-hours). As overall IHCA survival has improved over time, it remains unknown whether survival differences between on-hours and off-hours have changed. Objectives: This study sought to examine temporal trends in survival differences between on-hours and off-hours IHCA. Methods: We identified 151,071 adults at 470 U.S. hospitals in the Get with the Guidelines–Resuscitation registry during 2000 to 2014. Using multivariable logistic regression with generalized estimating equations, we examined whether survival trends in IHCA differed during on-hours (Monday to Friday 7:00 AM to 10:59 PM) versus off-hours (Monday to Friday 11:00 PM to 6:59 AM, and Saturday to Sunday, all day). Results: Among 151,071 participants, 79,091 (52.4%) had an IHCA during off-hours. Risk-adjusted survival improved over time in both groups (on-hours: 16.0% in 2000, 25.2% in 2014; off-hours: 11.9% in 2000, 21.9% in 2014; p for trend <0.001 for both). However, there was no significant change in the survival difference over time between on-hours and off-hours, either on an absolute (p = 0.75) or a relative scale (p = 0.059). Acute resuscitation survival improved significantly in both groups (on-hours: 56.1% in 2000, 71% in 2014; off-hours: 46.9% in 2000, 68.2% in 2014; p for trend <0.001 for both) and the difference between on-hours and off-hours narrowed over time (p = 0.02 absolute scale, p < 0.001 relative scale). In contrast, although post-resuscitation survival also improved over time in both groups (p for trend < 0.001 for both), the absolute and relative difference persisted. Conclusions: Despite an overall improvement in survival, lower survival in IHCA during off-hours compared with on-hours persists.