Apolipoprotein E4 (apoE4) plays a major role in the pathogenesis of Alzheimer's disease. Brain amyloid-β (Aβ) accumulation depends on age and apoE isoforms (apoE4 > apoE3) both in humans and in transgenic mouse models. Brain apoE levels are also isoform dependent, but in the opposite direction (apoE4 < apoE3). Thus, one prevailing hypothesis is to increase brain apoE expression to reduce Aβ levels. To test this hypothesis, we generated mutant human amyloid precursor protein transgenic mice expressing one or two copies of the human APOE3 or APOE4 gene that was knocked in and flanked by LoxP sites. We report that reducing apoE3 or apoE4 expression by 50% in 6-month-old mice results in efficient Aβ clearance and does not increase Aβ accumulation. However, 12-month-old mice with one copy of the human APOE gene had significantly reduced Aβ levels and plaque loads compared with mice with two copies, regardless of which human apoE isoform was expressed, suggesting a gene dose-dependent effect of apoE on Aβ accumulation in aged mice. Additionally, 12-month-old mice expressing one or two copies of the human APOE4 gene had significantly higher levels of Aβ accumulation and plaque loads than age-matched mice expressing one or two copies of the human APOE3 gene, suggesting an isoform-dependent effect of apoE on Aβ accumulation in aged mice. Moreover, Cre-mediated APOE4 gene excision in hippocampal astrocytes significantly reduced insoluble Aβ in adult mice. Thus, reducing, rather than increasing, apoE expression is an attractive approach to lowering brain Aβ levels.