Background: Brain changes involving the white matter (WM), often an indication of cerebrovascular pathology, are frequently seen in patients with mild cognitive impairment (MCI) and Alzheimer disease (AD). Few studies have examined possible cognitive domain-or group-specific cognitive effects of WM pathology in old age, MCI, and AD. Objective: Our purpose was to examine the relationship between WM hyperintensities (WMH), a typical marker for WM pathology, and cognitive functioning in healthy old age and pathological aging using quantified MRI data. Methods: We utilized multidomain neuropsychological data and quantified MRI data from a sample of 42 cognitively healthy older adults and 44 patients with MCI/AD (total n = 86). Results: After controlling for age and education, WMH in the temporal and parieto-occipital lobes was associated with impairments in processing speed and parieto-occipital pathology with verbal memory impairment in the whole sample. Additionally, temporal WMH was associated with impaired processing speed in the patient group specifically. Conclusions: WM pathology is strongly associated with impaired processing speed, and our results indicate that these impairments arise from WMH in the temporal and parieto-occipital regions. In MCI and AD patients with temporal WMH, processing speed impairments are especially prominent. The results of this study increase our knowledge of cognitive repercussions stemming from temporal and/or parieto-occipital WM pathology in healthy and pathological aging.