Individuals with ASD often display externalizing behaviors, which have been associated with lower quality of life in adulthood. Social difficulties have been hypothesized to underlie externalizing problems among individuals with ASD (Klin and Volkmar in "Asperger Syndrome," 340-366, 2000), but this has never been tested empirically. We examined whether socialization abilities predicted externalizing problems assessed by parent report in a group of 29 individuals with ASD (age range 7-16 years) and 29 TD individuals matched for IQ, age, and gender. Socialization scores accounted for 50% of the variance in externalizing behaviors among individuals with ASD, but not in TD children. These findings have implications for intervention, and suggest that targeting social difficulties might provide a better means to addressing externalizing problems.