Fear of strangers is a developmental milestone in childhood that encompasses behavioral inhibition and decreased novelty seeking. Children with attention deficit/hyperactivity disorder (ADHD) often exhibit fearless and impulsive behaviors, similar to those observed in children with atypically low levels of stranger fear. It is currently unknown whether these behaviors share common underlying biological mechanisms. Polymorphisms in the dopamine receptor 4 gene (DRD4) have been implicated in the risk for developing ADHD symptoms in childhood. Here we investigate whether (1) DRD4 variable number tandem repeats (VNTRs) are associated with both stranger fear and ADHD symptoms, and (2) stranger fear in preschoolers mediates the link between DRD4 VNTRs and ADHD in later childhood. Stranger fear was observed in a large sample (N=589) of 3-year-old Caucasian children and ADHD symptoms were assessed by a validated, mother-rated questionnaire at 6 years. We found evidence that longer DRD4 variants were associated with increased ADHD symptoms at 6 years, and that this relationship was partially mediated by lower levels of observed stranger fear at 3 years. Our results suggest a common underlying neurobiological mechanism in the association between low stranger fear and ADHD symptoms; variation in DRD4 may be an important contributor to this mechanism.