Attention deficit/hyperactivity disorder (ADHD) is the most frequently occurring neuropsychiatric disorder in childhood with an etiology that is not fully understood. A number of reviews that have addressed the neurobiology of ADHD have focused on imaging and genetics. Relatively little attention has been given to factors/mechanisms involved in the brain dysfunction. We suggest that changes in cellular immunity may be involved. Neopterin is a good indicator of cellular immunity, and we evaluated serum levels of neopterin in patients with ADHD. The study group consisted of 49 patients with ADHD. An age- and gender-matched control group was composed of 31 healthy subjects. Venous blood samples were collected, and the levels of neopterin were measured. The levels of neopterin were significantly higher in ADHD than in the comparison subjects. Cellular immunity may have a role in the etiopathogenesis of ADHD.