The aim of this study was to compare the vocabulary of boys with behavioral disorders (N = 34; age range: 10-13 years) and boys from the general population (N = 34; age range: 10-13 years), as well as examine the relationship between vocabulary and aggressive behavior and problems in their relationships with peers. Parents and caregivers assessed the child’s aggressive behavior and problems in relationships with peers, while data on the child’s vocabulary, cognitive abilities, and executive functions (working memory) were collected using a standardized intelligence test. By comparing the two groups, boys with behavioral disorders had a significantly poorer vocabulary than boys of the same age in the general population, and this difference could not be explained by the child’s cognitive abilities, or executive functions. In addition, poor vocabulary was significantly associated with peer relationship problems, as well as reactive, but not proactive, aggressive behavior. The findings have particular implications in the areas of early intervention for high-risk boys, as well as communication with children in the social welfare and justice systems.