Microglia are the principal immune cells of the brain. In Alzheimer disease, these brain mononuclear phagocytes are recruited from the blood and accumulate in senile plaques. However, the role of microglia in Alzheimer disease has not been resolved. Microglia may be neuroprotective by phagocytosing amyloid-β (Aβ), but their activation and the secretion of neurotoxins may also cause neurodegeneration. Ccr2 is a chemokine receptor expressed on microglia, which mediates the accumulation of mononuclear phagocytes at sites of inflammation. Here we show that Ccr2 deficiency accelerates early disease progression and markedly impairs microglial accumulation in a transgenic mouse model of Alzheimer disease (Tg2576). Alzheimer disease mice deficient in Ccr2 accumulated Aβ earlier and died prematurely, in a manner that correlated with Ccr2 gene dosage, indicating that absence of early microglial accumulation leads to decreased Aβ clearance and increased mortality. Thus, Ccr2-dependent microglial accumulation plays a protective role in the early stages of Alzheimer disease by promoting Aβ clearance.