The presence of Aβ_pE3 (N-terminal truncated Aβ starting with pyroglutamate) in Alzheimer’s disease (AD) has received considerable attention since the discovery that this peptide represents a dominant fraction of Aβ peptides in senile plaques of AD brains. This was later confirmed by other reports investigating AD and Down’s syndrome postmortem brain tissue. Importantly, Aβ_pE3 has a higher aggregation propensity, and stability, and shows an increased toxicity compared to full-length Aβ. We have recently shown that intraneuronal accumulation of Aβ_pE3 peptides induces a severe neuron loss and an associated neurological phenotype in the TBA2 mouse model for AD. Given the increasing interest in Aβ_pE3, we have generated two novel monoclonal antibodies which were characterized as highly specific for Aβ_pE3 peptides and herein used to analyze plaque deposition in APP/PS1KI mice, an AD model with severe neuron loss and learning deficits. This was compared with the plaque pattern present in brain tissue from sporadic and familial AD cases. Abundant plaques positive for Aβ_pE3 were present in patients with sporadic AD and familial AD including those carrying mutations in APP (arctic and Swedish) and PS1. Interestingly, in APP/PS1KI mice we observed a continuous increase in Aβ_pE3 plaque load with increasing age, while the density for Aβ_1-x plaques declined with aging. We therefore assume that, in particular, the peptides starting with position 1 of Aβ are N-truncated as disease progresses, and that, Aβ_pE3 positive plaques are resistant to age-dependent degradation likely due to their high stability and propensity to aggregate.