AbstractThe objective was to examine whether a common polymorphism in the dopamine D4 receptor gene (DRD4) might be a potential biomarker for behavioral variation within the autism spectrum disorder clinical phenotype. Children (N = 66) were evaluated with a validated mother‐ and teacher‐completed DSM‐IV‐referenced rating scale. Partial eta‐squared (ηp2) was used to gauge the magnitude of group differences: 0.01−0.06 = small, 0.06−0.14 = moderate and > 0.14 = large. Children who were 7‐repeat allele carriers had more severe oppositional defiant disorder behaviors according to mothers’ (ηp2 = 0.10) and teachers’ (ηp2 = 0.06) ratings than noncarriers, but the latter was marginally significant (P = 0.07). Children who were 7‐repeat allele carriers also obtained more severe maternal ratings of tics (ηp2 = 0.07) and obsessions–compulsions (ηp2 = 0.08). Findings for maternal ratings of separation anxiety were marginally significant (P = 0.08, ηp2 = 0.05). Analyses of combined DRD4 and dopamine transporter gene (DAT1) genotypes approached significance (P = 0.05) for teachers’ ratings of oppositional behavior and mothers’ ratings of tics. DRD4 allelic variation may be a prognostic biomarker for challenging behaviors in children with autism spectrum disorder, but these exploratory findings remain tentative pending replication with larger independent samples.