The principal characteristics of ADHD are inattention, hyperactivity, and impulsivity. These symptoms appear early in a child's life. Because many normal children may have these symptoms, but at a low level, or the symptoms may be caused by another disorder, it is important that the child receive a thorough examination and appropriate diagnosis by a well-qualified professional.
source:
National Institute of Mental Health
http://www.theaustralian.news.com.au/common/story_page/0,5744,16393063%255E30417,00.html