• A reduction of 30% in the Hyperactive-Impulsive (HI) subscale scores of the Conners Rating Scale-Revised, which indicates an improvement in both efficacy and a significant clinical improvement in the child. [ Time Frame: 7 to 13 weeks ]
  

• Decrease in ADHD specific symptom severity and impairment [ Time Frame: 7 to 13 weeks ]
  

Inclusion Criteria:

• Child must be 36-69 months old at the time of screening to insure that the child can complete the short-term safety and efficacy trial before the age of 6 years.
• Child must have categorical and dimensional evidence of clinically significant ADHD symptoms in multiple settings that have been present for at least six months.
• ADHD symptoms must cause significant functional impairment in the child. Child must meet impairment scale threshold based on Children's Global Assessment Scale (C-GAS, Shaffer, Gould, Brasic et al., 1983) score < 60.
• Child must have resided with primary caretaker for at least 6 months prior to the screening.
• Child's heart rate must be within the 98th percentile and blood pressure within the 95th percentile by age and gender and oral temperature values within the normal range.

Exclusion Criteria:

• Child with prior failed treatment with an adequate trial of ATMX (defined by a minimum of 1.8 mg/kg/day for 4 weeks) or known hypersensitivity to ATMX. Prior failure on stimulants or other psychotropics will not be the sole reason for exclusion.
• Child is taking MAOI or there have been less than 2 weeks since it was discontinued.
• Concurrent treatment with other medications that have CNS effects or that affect performance (e.g., antidepressants, antipsychotics, alpha-agonists, adrenergic blockers, lithium carbonate, sedating antihistamines, decongestant or sympathomimetics). Child should be off of previous psychotropic medications for a minimum duration of one week for stimulants, clonidine or guanfacine; two weeks for bupropion, tricyclic antidepressants, venlafaxine, SSRIs (except fluoxetine and citalopram), valproate; and three weeks for fluoxetine, citalopram, and neuroleptics.
• Child has narrow angle glaucoma.
• Child who has a major medical condition that would interfere with involvement in the study or would be affected negatively by ATMX (i.e., heart disease, high blood pressure, glaucoma, untreated or unstable hyperthyroidism, uncontrolled seizure disorder, or illnesses that would require hospitalization).
• Child with co-morbid psychiatric diagnoses of Major Depression, Bipolar Disorder, a psychotic disorder, or other psychiatric disorders in addition to ADHD that requires concurrent treatment with additional/alternative medication.
• Current history of physical, sexual, or emotional abuse.
• The patient has taken an investigational drug within the last 30 days.