White matter changes are often associated with stroke, risk factors for stroke, and dementia. From a theoretical point of view, they may be associated with an increased risk of pre- or poststroke dementia because (i) they are linked with subtle cognitive decline, which may add to the consequences of the stroke lesions and of associated Alzheimer pathology; and (ii) they indicate an increased risk of stroke recurrence. The aim of this study was to evaluate the contribution of white matter changes to pre- and poststroke dementia. The relationship between preexisting dementia and white matter changes was evaluated in the Lille stroke-dementia cohort. We assessed the cognitive functioning prior to stroke in 202 consecutive patients with ischemic or hemorrhagic stroke, by means of the Informant Questionnaire on Cognitive Decline in the Elderly (IQCODE). We classified in the dementia group patients with IQCODE scores of 104 or more. White matter changes were rated on CT with the Blennow's rating scale. Thirty-three of 202 patients were demented before stroke (16.3%; 95% confidence interval: 11.2-21.4); the logistic regression analysis found that female sex, family dementia, white matter changes, and cerebral atrophy were independently associated with prestroke dementia. White matter changes were also associated with an increased risk of poststroke dementia, 2 years after stroke onset. Thus, white matter changes contribute to dementia occurring in stroke patients.