This study analyzed whether platelet amyloid beta-protein precursor (AbetaPP) isoform ratio correlates with cognition or cognitive decline in patients with Alzheimer's disease (AD). Platelet AbetaPP isoform ratio was measured, and cognitive assessment was performed using the Mini-Mental State Examination (MMSE) in 66 AD patients at baseline (T0) and in 29 of these patients in a one-year follow-up (T1). There was a significant correlation between the AbetaPP isoform ratios and MMSE scores in the 66 AD patients at T0. The T1 subjects were divided into two groups: 12 "no decliners" (MMSE score, T1-T0 > or = 0) and 17 "decliners" (MMSE score, T1-T0 < 0). The decliners group showed a significantly greater reduction of AbetaPP isoform ratio from T0 to T1 than the no decliners group. However, the decline of the ratio did not correlate with the decline of MMSE score. These findings indicate that AbetaPP isoform ratio correlates with cognition, and reduction in this ratio may be a marker for cognitive decline in AD patients.