Possible precursors of serious aggression were identified in toddlers' use of force against peers. Instances of grabbing objects and hitting peers were recorded in a sample of 66 British 18- to 30-month-olds, observed at home with familiar peers and seen again 6 months later. Mothers rated aggressiveness in the context of other personality traits. Girls and boys did not differ in average levels of aggression, nor were they rated differently by the mothers. However, the observed rate of hitting peers and mothers' ratings of aggressiveness were stable over 6 months for girls, but not for boys. Toddlers who were especially sensitive to peers' possible intentions hit their peers more often. They were also more likely to use force proactively, 6 months later.