BACKGROUND AND PURPOSE - Leukoaraiosis (LA) might interrupt intra- and interhemispheric communication and thus induce cognitive impairments and dementia. It remains unclear, however, if there is a volume threshold of LA that is needed before either the signs of dementia and/or a specific pattern of neuropsychological impairment become manifest. Roman et al has suggested that 25% of white matter may need to be involved before white matter alterations influence the clinical signs associated with dementia. The purpose of this study is to ascertain the threshold of MRI-LA as measured with a visual rating scale needed to induce specific patterns of neuropsychological impairment associated with dementia. METHODS - One hundred fifteen patients with dementia received a comprehensive neuropsychological examination and the severity of MRI-LA was measured using a 40-point LA scale. RESULTS - Patients were categorized into low (mean LA=4.21±2.92; 3.22%-17.82%), moderate (mean LA=12.58±2.54; 25.01%-37.80%), and severe (mean LA=22.36±4.04; 45.80%-66.00%) LA groups. Patients in the mild LA group obtained markedly lower scores on tests of episodic memory compared with working memory, a neuropsychological profile often associated with Alzheimer disease. Patients with moderate LA displayed equal impairment on neuropsychological tests. Patients in the severe LA group obtained significantly lower scores on tests of working memory as compared with episodic memory. CONCLUSIONS - These data provide evidence that a threshold of moderate MRI-LA as measured with a visual rating scale is associated with greater and/or equal impairment on tests of working memory versus episodic memory and provides a benchmark to assess the effect of MRI-LA on the clinical presentation of dementia. © 2008 American Heart Association, Inc.