Research on aggression over the past two decades has focused on gene-environment interaction models to explain the relative contribution of each to this behavioral phenotype in various clinical populations. Recent investigations suggest a link between aggression in people with intellectual disabilities the functionality of the serotonin transporter. The aims in this study were to examine the possible association of the STin2 and/or the 5-HTTLPR serotonin transporter polymorphisms in adult males with and without intellectual disabilities, and to examine the association of these polymorphisms with aggression in people with intellectual disabilities. DNA samples and behavioral records were obtained from adult males with intellectual disabilities, distinguished only by the presence or absence of aggression. No association was found between either transporter polymorphism for aggression. However, the long 5-HTTLPR allele, and not the short allele or the heterozygous state, was associated with the severity of aggression. The association with aggression appears to be genetically complex, suggesting there may be other genes, interactions between genes, and/or environmental relations occasioning aggression in people with intellectual disabilities.