An altered pattern of platelet amyloid precursor protein (APP) forms, consisting of a reduced ratio of the upper (130 kDa) to the lower (110 to 106 kDa) immunoreactivity band (APPr), is associated with Alzheimer disease (AD), although in the early stages of AD this pattern shows high variability. To explore whether APPr values at baseline may predict the rate of cognitive decline, we evaluated patients with mild AD before and after 1 year of treatment with acetylcholinesterase inhibitors. Lower APPr at baseline was the only predictor of progressive cognitive decline, suggesting that this biomarker might be a useful indicator of prognosis for patients with AD.