Background Previous investigations have demonstrated that an MspI polymorphism at the adrenergic α2A receptor gene (ADRA2A) is associated with severity of attention-deficit/hyperactivity disorder (ADHD) inattentive symptoms in clinical samples composed mainly of subjects with ADHD, combined type. This study aimed to investigate the association between this ADRA2A polymorphism and attention-deficit/hyperactivity disorder–inattentive type (ADHD-I) in a nonreferred sample. Methods In a case–control study, we assessed a sample of 100 children and adolescents with ADHD-I and 100 non-ADHD controls. Cases and controls were matched by gender and age and were screened by using teacher reports in a revised version of the Swanson, Nolan, and Pelham rating scale at 12 schools. Psychiatric diagnoses were derived through structured diagnostic interviews. Results Homozygous subjects for the G allele at the ADRA2A had significantly higher odds ratio (OR) for ADHD-I than did those with other genotypes (CC + CG genotypes), even after adjusting for potential confounders ( p = .02; OR = 3.78; 95% confidence interval = 1.23–11.62). In family-based analyses, no significant associations were detected. Conclusions Our results suggest that the ADRA2A may be associated with ADHD-I, replicating previous findings from clinical samples that have suggested the importance of this gene for the dimension of inattention. In addition, these results support the role of the noradrenergic system in ADHD.