We report a 79-year-old woman with possible Alzheimer's disease and confluent periventricular white matter hyperintensities on magnetic resonance imaging in whom postmortem analysis unexpectedly demonstrated no periventricular demyelination or cerebral arteriosclerosis. However, astrocytes in the periventricular white matter exhibited clasmatodendrosis, defined as cytoplasmic swelling and vacuolation of astroglia, with beading of their dendrites. This finding represents a previously unrecognized correlate of periventricular white matter hyperintensities. Ann Neurol 2002;52:378-381