BACKGROUND AND PURPOSE - Besides cerebrovascular disease, medial temporal lobe atrophy (MTA), a neuroimaging finding suggestive of degenerative pathology, has been shown in vascular dementia (VaD). However, it is unknown to what extent MTA contributes to the pattern of cognitive impairment observed in VaD. Therefore, our purpose was to investigate the relative contribution of cerebrovascular disease and MTA to cognitive impairment in patients fulfilling diagnostic criteria for VaD. METHODS - We examined 590 patients (374 men; mean age, 73 years; standard deviation, 8) with probable VaD according to the National Institute of Neurological Disorders and Stroke-Association Internationale pour la Recherche et l'Enseignement en Neurosciences criteria at inclusion into a multicenter clinical trial. Cerebrovascular disease and the degree of MTA were evaluated by using MRI. Cognitive testing included the Mini-Mental State Examination, and the vascular dementia assessment scale. RESULTS - On the basis of the operational definitions for the neuroimaging part of the National Institute of Neurological Disorders and Stroke-Association Internationale pour la Recherche et l'Enseignement en Neurosciences criteria, 485 (82.2%) patients had small vessel VaD and 153 (25.9%) had large vessel VaD. More than half (59.8%) of the patients had considerable MTA. Multiple linear regression analyses revealed that after correction for sex, age, education, and duration of dementia, neuropsychological tests showed that patients with higher grades of MTA or large vessel VaD had significantly worse general cognitive and executive functioning, whereas associations with small vessel disease were restricted to worse executive functioning. CONCLUSIONS - Both MTA and large vessel disease contribute to global cognitive impairment in VaD. Small vessel disease contributes to executive dysfunction.