Measurements of hippocampal formation atrophy using MRI have been useful in distinguishing demented patients with a diagnosis of probable Alzheimer's disease from cognitively normal controls. To determine whether there is a similar relationship between hippocampal size and dementia in elderly patients suspected of normal pressure hydrocephalus (NPH), the authors obtained mini-mental status examination (MMSE) scores and MRI measurements of hippocampal size and CSF volume on 16 elderly patients whose severe ventriculomegaly and unexplained gait impairment made NPH a probable diagnosis. Hippocampal size correlated strongly with MMSE score (r = 0.75, p < 0.001); no significant MMSE correlation was found for ventricular CSF volume or extra-ventricular/ventricular CSF ratio. It was concluded that hippocampal atrophy is associated with severe cognitive dysfunction in many elderly patients with a diagnosis of NPH. As a hypothesis for further investigation, the detection of such atrophy may help identify cases where the presence of a pathology of Alzheimer's disease complicates the diagnosis of NPH.