BACKGROUND:Recent reports have suggested a pathophysiological role of brain-derived neurotrophic factor (BDNF) in attention deficit-hyperactivity disorder (ADHD). We evaluated the plasma levels of BDNF in patients with ADHD. METHODS:Plasma BDNF levels were measured in 41 drug naive ADHD patients and 107 normal controls. The severity of ADHD symptoms was determined by patient scores on the ADHD rating scale (ARS) and the computerized ADHD diagnostic system (ADS). RESULTS:ANCOVA with age and gender as covariates showed that the mean plasma BDNF levels were significantly higher in ADHD patients than in normal controls (F=16.968, p<0.001). There were also significant differences in plasma BDNF levels of ADHD patients and those of normal controls for males and females (Mann-Whitney U-test, p=0.001 and 0.041, respectively). We also found a significant correlation between plasma BDNF levels and omission errors in ADS outcome-variable T-scores (p<0.001). CONCLUSIONS:Our study suggests that there is an increase of plasma BDNF levels in untreated ADHD patients, and that plasma BDNF levels had a significant positive correlation with the severity of inattention symptoms. Further studies are required to elucidate the source and role of circulating BDNF in ADHD.