Abnormal right prefrontal cortex activity is one of the key and most replicable neural findings in attention deficit hyperactivity disorder, but only limited attempts were made to directly alter prefrontal activity as a possible treatment. Here we evaluated whether targeting this region with multiple sessions of repetitive transcranial magnetic stimulation, will affect clinical symptoms in adult patients. In addition, we used EEG to characterize electrophysiological alterations induced by stimulation and to correlate between baseline neuronal activity and clinical response. Forty-three, psychostimulants-free patients, were randomized to receive either Real, Active Control, or Sham treatment (13 females, ages ranging 21-46; n=15, 14, 14, respectively), and underwent three weeks of daily high-frequency (18 Hz) stimulation sessions. We found that only Real treatment resulted in significant improvement of attention and hyperactivity symptoms (η2p=0.34; Cohen's d(against Sham)=0.96; p=0.00085), which were accompanied by (η2p=0.45; p=0.00057), and correlated with (r=0.85, p=0.001), enhancement of cognitive performance. Furthermore, based on EEG recorded during the first treatment session we established a novel, theoretically apposite prognostic biomarker which was observed with close physical resemblance to the stimulation area and highly predicted the magnitude of the clinical outcome (r=0.92, p=0.0001). This biomarker is composed from the ratio between Alpha and Low-gamma frequency bands and is suggested to reflect the responsiveness of the cortex to the stimulation. Taking together, this is the first comparative and controlled study directly indicating safety and effectiveness of TMS for treatment of adult ADHD patients, while allowing pre-check for selection of optimal candidates.