This 4 arm study will assess the efficacy and safety of RO5313534 (MEM3454) versus placebo added to donepezil, in patients with mild to moderate Alzheimer's disease. Following a screening period, patients will be randomized to one of 4 treatments (placebo, or RO5313534 1mg, 5mg or 15mg po daily) with background therapy of donepezil (5mg or 10mg).The anticipated time on study treatment is 6 months, and the target sample size is 100-500 individuals.

Inclusion Criteria:

• adult patients, >/=50 years of age;
• probable Alzheimer's disease;
• MMSE score at screening of 13-22;
• under stable donepezil treatment given at a fixed dose of 5 or 10mg daily for >=4 months prior to baseline;
• not requiring nursing home care, but looked after by a caregiver/carer.

Exclusion Criteria:

• dementia due to condition other than Alzheimer's disease;
• other significant neurological disorder;
• untreated/non-stabilized major depressive disorder;
• bipolar disorder, schizophrenia, or any other serious psychiatric condition.