A functional polymorphism in the promoter of the gene encoding monoamine oxidase A has been associated with problem behavior in various populations. We examined the association of MAOA alleles in adult males with intellectual/developmental disabilities with and without established histories of problem behavior. These data were compared with a gender, ethnicity, and age-matched contrast sample. About 43% (15/35) of adults with intellectual/developmental disabilities and problem behavior possessed the low-efficiency version of the MAOA gene. In comparison, 20% (7/35) of adults with intellectual/developmental disabilities and no problem behavior and 20% (7/35) of the contrast group had the short-allele MAOA polymorphism. Therefore, a common variant in the MAOA gene may be associated with problem behavior in adults with intellectual/developmental disabilities.