Hostile attributions are acknowledged as an important part of maladaptive social information-processing that results in aggression in children. Literature suggests the largest effect size between aggressive behavior and hostile attributions is found when hostile attributions are measured using staged situations rather than commonly used vignette-based measures (Orobio de Castro et al., 2002), but few, if any, studies have investigated hostile attributions as they occur in real-time. The current study uses an interactive, video racing game along with a verbal and nonverbal response procedure to measure hostile attributions while children are playing against a presumed peer. A sample of 75 children between the ages of 10 and 13 played the game while their parents were asked to rate their child's reactive and proactive aggression. It was expected that the new real-time measure of hostile attributions would predict reactive aggression, but not proactive aggression. The predictive ability of the new measure was compared with that of the traditional, vignette-based measure of hostile attributions. Participants played a computer video game created for this study by racing their car against a presumed peer, though the opponent's car was actually computer-operated and reprogrammed to ambiguously provoke participants by crashing into their car. A visual prompt appeared on screen to elicit real-time attribution responses, either verbally or nonverbally, after each of eight crashes and at eight non-crash moments. Participants' hostile attributions using the nonverbal and verbal response procedures were significantly positively related to parent-rated reactive aggression, when controlling for proactive aggression. Multiple regression indicated that the real-time measure was a significant predictor of reactive aggression, while the vignette-based measure was not. The results of this study illustrate how hostile attributions in real-time occur quickly and simultaneously with social interaction, which differs from the deliberative, controlled appraisal process described in previous literature. These findings can be used to identify specific appraisal deficits in children at risk for aggressive behavior, which could enhance preventive interventions by more precisely targeting these cognitions and more effectively preventing aggression.