The purpose of this study is to determine if CLONICEL (clonidine HCl sustained release) is a safe and effective add-on to psychostimulant therapy in children and adolescents with attention deficit hyperactivity disorder (ADHD).

Inclusion Criteria:

• Age 6-17 years, inclusive
• Diagnosis of ADHD
• Currently on a stable psychostimulant regimen for ADHD
• Lack of adequate response to stable psychostimulant regimen
• Ability to swallow tablets

Exclusion Criteria:

• Clinically significant illnesses or abnormalities upon evaluation
• Conduct Disorder
• Intolerance to clonidine
• History of seizures or syncope