Attention deficit/hyperactivity disorder (ADHD) is the most commonly diagnosed neurobehavioral disorder of childhood. The etiopathogeny of ADHD has not been totally defined. Recent reports have suggested a pathophysiological role of neurotrophins in ADHD. In this study, we evaluated serum levels of nerve growth factor (NGF) in patients with ADHD. The sample population consisted of 44 child or adolescent patients diagnosed with ADHD according to DSM-IV criteria; 36 healthy subjects were included in the study as controls. Venous blood samples were collected, and NGF levels were measured. The mean serum NGF levels of the ADHD patients were significantly higher than those of the controls. Age and gender of the patients were not correlated with serum NGF levels. There were no significant differences in NGF levels among the combined and predominantly inattentive subtypes of ADHD. Our study suggests that there are higher levels of serum NGF in drug naive ADHD patients, and that increased levels of NGF might have an important role in the pathophysiology of ADHD. (C) 2013 Elsevier Ireland Ltd. All rights reserved.