Apolipoprotein E (apoE) genotype has a major influence on the risk for Alzheimer disease (AD). Different apoE isoforms may alter AD pathogenesis via their interactions with the amyloid β-peptide (Aβ). Mice lacking the lipid transporter ABCA1 were found to have markedly decreased levels and lipidation of apoE in the central nervous system. We hypothesized that if Abca1-/- mice were bred to the PDAPP mouse model of AD, PDAPP Abca1-/- mice would have a phenotype similar to that of PDAPP Apoe+/- and PDAPP Apoe-/- mice, which develop less amyloid deposition than PDAPP Apoe+/+ mice. In contrast to this prediction, 12-month-old PDAPP Abca-/- mice had significantly higher levels of hippocampal Aβ, and cerebral amyloid angiopathy was significantly more common compared with PDAPP Abca1+/+ mice. Amyloid precursor protein (APP) C-terminal fragments were not different between Abca1 genotypes prior to plaque deposition in 3-month-old PDAPP mice, suggesting that deletion of Abca1 did not affect APP processing or Aβ production. As expected, 3-month-old PDAPP Abca1-/- mice had decreased apoE levels, but they also had a higher percentage of carbonate-insoluble apoE, suggesting that poorly lipidated apoE is less soluble in vivo. We also found that 12-month-old PDAPP Abca1 -/- mice had a higher percentage of carbonate-insoluble apoE and that apoE deposits co-localize with amyloid plaques, demonstrating that poorly lipidated apoE co-deposits with insoluble Aβ. Together, these data suggest that despite substantially lower apoE levels, poorly lipidated apoE produced in the absence of ABCA1 is strongly amyloidogenic in vivo.