The purpose of this study is to test if the investigational medication ABT-089 is a safe and effective treatment for Alzheimer's disease.

Inclusion Criteria:

• Patient has mild to moderate Alzheimer's disease
• Patient has been receiving a stable dose of an acetylcholinesterase inhibitor
• Patient has a MMSE score between 12 and 26
• Patient has a MHIS score of less than or equal to 4
• Females must be postmenopausal for at least two years or surgically sterile
• Patient has an identified, reliable, caregiver who will support him/her to ensure compliance with treatment and will accompany them to each outpatient visit

Exclusion Criteria:

• Patient is living in a nursing home
• Patient has a history of any significant neurologic disease other than Alzheimer's disease
• Patient has any uncontrolled medical illness
• Patient uses non-prescribed drugs of abuse or has a history of drug or alcohol abuse/dependence