Apolipoprotein E epsilon4 is an independent risk factor for Alzheimer's disease (AD) and is the main constituent of high-density lipoprotein (HDL) as a source of cholesterol in the brain. ATP-binding cassette transporter G4 (ABCG4) is one of the membrane cholesterol transporter which is implicated in HDL-mediated cholesterol efflux, but its precise localization and function in the brain has been unclear. In AD brain, ABCG4 protein was highly expressed in microglial cell that was closely located to senile plaques, whereas in non-neurological cases positive cells were not seen in cortical and nigral tissues. As well as the ABCG4 protein, ABCG4 mRNA signal was detected in microglial cell closely located to senile plaque of AD brain by in situ hybridization histochemistry. These results suggest that upregulated ABCG4 in microglia may accelerate the lipidation of apoE and HDL in the AD brain. This is the first report to show that ABCG4 is highly expressed in microglia on AD brain.