Mutations in the X-linked adrenoleukodystrophy (X-ALD) protein cause accumulation of unbranched saturated very-long-chain fatty acids, particularly in brain and adrenal cortex. In humans, the genetic defect causes progressive inflammatory demyelination in the brain, where very-long-chain fatty acids accumulate within phospholipid fractions such as lysophosphatidylcholine.To address mechanisms of inflammation, we studied microglial activation in human ALD (10 autopsies) and lysophosphatidylcholine (C24:0) injection into the parietal cortex of mice.Unexpectedly, we found a zone lacking microglia within perilesional white matter, immediately beyond the actively demyelinating lesion edge. Surrounding this zone we observed clusters of activated and apoptotic microglia within subcortical white matter. Lysophosphatidylcholine (C24:0) injection in mice led to widespread microglial activation and apoptosis.Our data suggest that the distinct mononuclear phagocytic cell response seen in cerebral X-ALD results, at least in part, from aberrant signaling to cognate receptors on microglia. Our findings support a hypothesis that microglial apoptosis in perilesional white matter represents an early stage in lesion evolution and may be an appropriate target for intervention in X-ALD patients with evidence of cerebral demyelination.