A case study on neurofeedback and cognitive behavior therapy (CBT) of an 11-year-old girl diagnosed with Attention Deficit Hyperactivity Disorder (ADHD) and oppositional defiant disorder (ODD) is presented. The quantitative electroencephalograms (QEEG) demonstrated a low-voltage electroencephalograms (EEG) with excess relative theta. After 31 sessions of theta/beta neurofeedback treatment, the client no longer met criteria for both ADHD and ODD. Her inattentive problems and hyperactive and impulsive behavior declined to normal levels, further evidenced by neuropsychological and event related potentials (ERP) improvements. Also oppositional and aggressive behavior improved dramatically. This case study demonstrates the possibility to treat clients with ADHD and comorbid disorders with combined neurofeedback and CBT. However, further controlled studies are required to replicate and extend these findings and investigate which effects are related to neurofeedback and which to CBT.