We studied emotional aspects of social information processing (SIP) and their specific relations with reactive and proactive aggression in 54 boys ages 7 to 13 who had been referred for aggressive behavior problems and a comparison group. Participants listened to vignettes concerning provocations by peers and answered questions concerning SIP, own and peer's emotions, and emotion regulation. Aggressive boys attributed more hostile intent, happiness, and less guilt; reported more anger; mentioned less adaptive emotion-regulation strategies; generated more aggressive responses; and evaluated aggressive responses less negatively than comparison boys. Hypothesized specific relations with reactive and proactive aggression were found, except for emotion regulation that was negatively related with both kinds of aggression. Potentially confounding effects of socially desirable answering, verbal intelligence, and recall of vignettes were controlled for.