Inclusion Criteria:

• Patients with a prior diagnosis of a pediatric malignancy (brain tumors, solid tumors, leukemia/lymphoma) who are in remission, and are > 3 years from initial diagnosis. Patients must have completed therapy a minimum of 6 months prior to study entry.
• Patients must have English as their primary language
• Patients must be >3 and less than or equal to 16 at the time of study enrollment
• Patients must fulfill the operational criteria for neurocognitive deficit and have an IQ > 50.
• Patients must be currently enrolled in a school or a learning environment where an adult familiar with the child's academic performance can provide ratings of that performance over the time of the trial.
• Patients must have wbc, hemoglobin and platelet parameters which fall within the norms for the patient's age.
• Patients must have adequate hepatic function (bilirubin less than or equal to 1.5 mg/dl and SGPT < 5x normal) and renal function (normal Cr for age/GFR greater than or equal to 70 ml/min/1.73m2)
• Patients must be able to swallow gel caps.
• Signed informed consent must be obtained according to institutional guidelines. When appropriate the patient will be included in all discussion in order to obtain verbal assent.
• Protocol and informed consent must be approved by the local IRB prior to any patient registration and reapproved every 12 months.

Exclusion Criteria:

• Patients who have evidence of a significant neurological abnormality prior to their cancer diagnosis that would affect neuro-behavioral development (such as genetic: Fragile X, Downs syndrome, or acquired disorders: closed head injury)
• Patients with a prior (pre-cancer) diagnosis of attention deficit hyperactivity disorder or major depression
• Patients who have a medical condition which would preclude the use of clonidine (medicated for hypertension, cardiac conduction disturbances, cerebrovascular disease, or chronic renal failure)
• Patients must not have received psychotropic medication (SSRIs, methylphenidate), anxiolytics, or hypnotics within 2 weeks of study entry.
• Patients who are receiving concurrent scheduled barbiturates or sedating drugs.
• Currently (within 6 months) known to abuse drugs or to be dependent on any drug including alcohol or with a positive urine drug screen.
• Women of childbearing age must not be pregnant or lactating. This group is excluded because of teratogenic potential of this agent demonstrated in rat and rabbit models.