DSM-IV Criteria for ADHD
I. Either A or B:

Six or more of the following symptoms of inattention have been present for at least 6 months to a point that is disruptive and inappropriate for developmental level:

Inattention

• Often does not give close attention to details or makes careless mistakes in schoolwork, work, or other activities.
• Often has trouble keeping attention on tasks or play activities.
• Often does not seem to listen when spoken to directly.
• Often does not follow instructions and fails to finish schoolwork, chores, or duties in the workplace (not due to oppositional behavior or failure to understand instructions).
• Often has trouble organizing activities.
• Often avoids, dislikes, or doesn't want to do things that take a lot of mental effort for a long period of time (such as schoolwork or homework).
• Often loses things needed for tasks and activities (e.g. toys, school assignments, pencils, books, or tools).
• Is often easily distracted.
• Is often forgetful in daily activities.

Six or more of the following symptoms of hyperactivity-impulsivity have been present for at least 6 months to an extent that is disruptive and inappropriate for developmental level:

Hyperactivity

• Often fidgets with hands or feet or squirms in seat.
• Often gets up from seat when remaining in seat is expected.
• Often runs about or climbs when and where it is not appropriate (adolescents or adults may feel very restless).
• Often has trouble playing or enjoying leisure activities quietly.
• Is often "on the go" or often acts as if "driven by a motor".
• Often talks excessively.

Impulsivity

• Often blurts out answers before questions have been finished.
• Often has trouble waiting one's turn.
• Often interrupts or intrudes on others (e.g., butts into conversations or games).
• Some symptoms that cause impairment were present before age 7 years.
• Some impairment from the symptoms is present in two or more settings (e.g. at school/work and at home).

There must be clear evidence of significant impairment in social, school, or work functioning.

The symptoms do not happen only during the course of a Pervasive Developmental Disorder, Schizophrenia, or other Psychotic Disorder. The symptoms are not better accounted for by another mental disorder (e.g. Mood Disorder, Anxiety Disorder, Dissociative Disorder, or a Personality Disorder).

Based on these criteria, three types of ADHD are identified:

ADHD, Combined Type: if both criteria 1A and 1B are met for the past 6 months

ADHD, Predominantly Inattentive Type: if criterion 1A is met but criterion 1B is not met for the past six months 

ADHD, Predominantly Hyperactive-Impulsive Type: if Criterion 1B is met but Criterion 1A is not met for the past six months.

American Psychiatric Association: Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition, Text Revision. Washington, DC, American Psychiatric Association, 2000.