Limited information is available concerning the specificity of the forms and functions of aggressive behavior exhibited by boys with fragile X syndrome (FXS). To investigate these relationships, we conducted indirect functional assessments of aggressive behavior exhibited by 41 adolescent boys with FXS and 59 age and symptom-matched controls with intellectual and developmental disability (IDD) and compared the data between groups. Results showed that boys with FXS were more likely to exhibit specific forms of aggressive behavior (i.e., scratching others and biting others) compared to controls, but the sources of reinforcement identified for aggression were similar across groups. Boys with FXS who were prescribed psychotropic medications were more likely to be older and to exhibit more forms of aggression. The implications for the treatment of aggressive behavior during this critical developmental period in FXS are discussed.