A small area of changed signal intensity in the hippocampus is often seen on magnetic resonance (MR) images of the brain in patients without specific clinical signs or symptoms. To ascertain its cause by means of histologic examination, this finding was evaluated retrospectively in 109 patients and correlated with findings in two human brain specimens. This area of change was typically round or curvilinear and 1-2 mm in diameter. Its location was between the hippocampus and dentate gyrus. The signal intensity was the same as that of cerebrospinal fluid (CSF) with all MR sequences used. The incidence of change in signal intensity was greater in elderly patients. Correlation with histologic findings showed that this area of change, a dilated perivascular space, was the residual cavity of the hippocampal sulcus. Whenever an area of CSF-like signal intensity has this shape and topographic features, the possibility of anatomic variation should be considered before the change in signal intensity is diagnosed as brain injury.