In a prospective magnetic resonance imaging and cognitive study of 38 demented patients and 15 control subjects, 11 of 27 patients with Alzheimer's disease and 8 of 11 patients with vascular dementia had significant periventricular hyperintensities. Memory and language testing in the early investigation of dementia is useful to distinguish patients with or without periventricular hyperintensities on magnetic resonance imaging. Patients without periventricular hyperintensities are worse on memory and conceptualization tests than patients with periventricular hyperintensities, who tend to be worse on comprehension and attention tests. These differences in cognitive pattern are present between patients with different pathogenesis who are otherwise matched for dementia severity. Language and some nonverbal cognitive deficits correlate with the extent of cortical and ventricular atrophy in Alzheimer's disease.