The epidemic of late life dementia, prominence of use of alternative medications and supplements, and initiation of efforts to determine how to prevent dementia have led to efforts to conduct studies aimed at prevention of dementia. The GEM (Ginkgo Evaluation of Memory) study was initially designed as a 5-year, randomized double-blind, placebo-controlled trial of Ginkgo biloba, administered in a dose of 120 mg twice per day as EGb761, in the prevention of dementia (and especially Alzheimer's disease) in normal elderly or those with mild cognitive impairment. The study anticipates 8.5 years of participant follow-up. Initial power calculations based on estimates of incidence rates of dementia in the target population (age 75+) led to a 3000-person study, which was successfully recruited at four clinical sites around the United States from September 2000 to June 2002. Primary outcome is incidence of all-cause dementia; secondary outcomes include rate of cognitive and functional decline, the incidence of cardiovascular and cerebrovascular events, and mortality. Following screening to exclude participants with incident dementia at baseline, an extensive neuropsychological assessment was performed and participants were randomly assigned to treatment groups. All participants are required to have a proxy who agreed to provide an independent assessment of the functional and cognitive abilities of the participant. Assessments are repeated every 6 months. Significant decline at any visit, defined by specific changes in cognitive screening scores, leads to a repeat detailed neuropsychological battery, neurological and medical evaluation and MRI scan of the brain. The final diagnosis of dementia is achieved by a consensus panel of experts. Side effects and adverse events are tracked by computer at the central data coordinating center and unblinded data are reviewed by an independent safety monitoring board. Studies such as these are necessary for this and a variety of other potential protective agents to evaluate their effectiveness in preventing or slowing the emergence of dementia in the elderly population.