Attention-Deficit Hyperactivity Disorder (abbreviated as ADHD) is characterized by high levels of inattention, being highly distractible, impulsive and restless, hyperactive or hypersensitive. Most demigods are labeled as suffering from ADHD, but it is actually a sign of their heightened senses and natural aptitude for battle. ADHD also gives demigods greater battlefield reflexes and the ability to see where their opponents will strike due to the tensing of their muscles. It is usually coupled with dyslexia in demigods.
In the series, ADHD is present everywhere from The Lightning Thief to The Last Olympian. Grover Underwood tells Percy that ADHD makes him inattentive to other things whenever he is in class. Grover also says that the instinct makes him want to get up and get outside to release that energy. Percy states that having ADHD makes a demigod very impatient, as Thalia Grace showed in The Last Olympian.
All introduced demigod characters, with the exception of Frank Zhang, have ADHD (all Frank got was lactose intolerance). Leo Valdez, on the other hand, has been said to be extremely ADHD (even for a demigod), but no mention has been made about him being dyslexic.
Frank Zhang is currently the only known demigod to not have ADHD, while Leo Valdez is the demigod with the most serious case of ADHD.