This study will investigate AQW051 in patients with either mild Alzheimer's disease or amnestic mild cognitive impairment. The effect on cognitive impairment will be measure using validated computerized tests which measure cognitive function. This study will also explore the safety and tolerability of AQW051 in these patients.

Inclusion Criteria:

• Willing and able to give written informed consent
• Meet the diagnostic criteria for either amnestic mild cognitive impairment (amnestic MCI) or mild Alzheimer's Disease (AD).
• Structural brain scan within the last 6 months prior to randomization that indicates no other underlying disease, in particular no evidence for vascular pathology except for normal age-related white matter/incidental white matter changes which is normal for this age group.
• Daily contact with a primary caregiver/partner

Exclusion Criteria:

• Immune therapy targeting Alzheimer beta amyloid within the last 12 months
• Institutionalized
• Disability that may prevent completion of all study requirements (e.g., blindness, deafness, or communication difficulty)
• Reported use of tobacco products in the previous 3 months or have a urine cotinine level greater than 500 ng/ml
• Past medical history of clinically significant electrocardiogram (ECG) abnormalities or a family history (grandparents, parents, and siblings) of prolonged QT-interval syndrome
• History or current diagnosis of conditions specified in the protocol.

Other protocol-defined inclusion/exclusion criteria may apply.