A polymorphism in the dopamine receptor 4 gene (DRD4) has been related to novelty seeking, Tourette's syndrome, and attention deficit hyperactivity disorder (ADHD). The variability is in a 48-bp repeat in exon 3 of the gene (a transmembrane region). This study examined the relation of the 7-repeat (i.e., high-risk) allele to questionnaire-based diagnoses of ADHD (both combined type and inattentive type). Several positive findings were obtained for ADHD-inattentive type. In an association test, the 7-repeat allele occurred more frequently in children with ADHD-inattentive type than in control children. In genetically discordant sibling pairs, the sibling with a greater number of 7-repeat alleles displayed more inattentive symptoms than his/her co-sibling with fewer 7-repeat alleles. For ADHD-combined type, the 7-repeat allele frequency was greater than that in the control sample. However, a quantitative transmission disequilibrium test yielded no significant linkage of the 7-repeat allele with hyperactive-impulsive symptoms. A categorical TDT yielded no significant findings, but the number of transmissions was small, especially for ADHD-inattentive type.