It is well known that violent video games increase aggression, and that stress increases aggression. Many violent video games can be stressful because enemies are trying to kill players. The present study investigates whether violent games increase aggression by inducing stress in players. Stress was measured using cardiac coherence, defined as the synchronization of the rhythm of breathing to the rhythm of the heart. We predicted that cardiac coherence would mediate the link between exposure to violent video games and subsequent aggression. Specifically, we predicted that playing a violent video game would decrease cardiac coherence, and that cardiac coherence, in turn, would correlate negatively with aggression. Participants (N = 77) played a violent or nonviolent video game for 20 min. Cardiac coherence was measured before and during game play. After game play, participants had the opportunity to blast a confederate with loud noise through headphones during a reaction time task. The intensity and duration of noise blasts given to the confederate was used to measure aggression. As expected, violent video game players had lower cardiac coherence levels and higher aggression levels than did nonviolent game players. Cardiac coherence, in turn, was negatively related to aggression. This research offers another possible reason why violent games can increase aggression—by inducing stress. Cardiac coherence can be a useful tool to measure stress induced by violent video games. Cardiac coherence has several desirable methodological features as well: it is noninvasive, stable against environmental disturbances, relatively inexpensive, not subject to demand characteristics, and easy to use. Aggr. Behav. 39:64-70, 2013. © 2012 Wiley Periodicals, Inc.