Enzyme-generated fragments of tau have been linked to neuronal death and may serve as serum biomarkers of cognitive loss. Two competitive ELISAs detecting an ADAM10-generated fragment (Tau-A) or a caspase-3-generated fragment (Tau-C) were measured in baseline serum samples from patients with mild to moderate Alzheimer's disease (AD) from a Phase III clinical trial, and correlated to change in the Alzheimer's Disease Assessment Scale-Cognitive subscale (ADAS-Cog11) and Clinical Dementia Rating-Sum of Boxes (CDR-SB) over a 64-week period using an MMRM-analysis. Relationship between the biomarkers and changes in ADAS-Cog11 score as a function of time were observed for Tau-C and change in ADAS-Cog11 (p = 0.06), and for Tau-A and change in CDR-SB (p = 0.04). The correlation of Tau-A/Tau-C ratio with cognitive change assessed by ADAS-Cog11 was even more significant (p < 0.006). These data indicate that measuring the balance between tau fragments in serum may provide a marker of the rate of progression of AD and warrant studies in larger cohorts