Amyloid-beta (Aβ) accumulation was evaluated with 2 [ 11C]Pittsburgh compound B (PiB) positron emission tomography scans about 2.5 years apart in 146 cognitively normal adults. Seventeen of 21 participants with initially elevated Aβ deposition demonstrated subsequent Aβ plaque growth (approximately 8.0% per year), and none reverted to a state of no Aβ deposits. Ten individuals converted from negative to positive PiB status, based on a threshold of the mean cortical binding potential, representing a conversion rate of 3.1% per year. Individuals with an Îμ4 allele of apolipoprotein E demonstrated increased incidence of conversion (7.0% per year). Our findings suggest that the major growth in Aβ burden occurs during a preclinical stage of Alzheimer disease (AD), prior to the onset of AD-related symptoms.