We examined the utility of illusory contours (ICs) for the differentiation of dementia with Lewy bodies (DLB) from Alzheimer's disease (AD). Thirty-five probable DLB patients, 35 probable AD patients controlled by age, years of education, and Mini-Mental State Examination (MMSE) score, and 30 cognitively normal subjects controlled by age and years of education underwent visuoperceptual examinations including ICs, pentagon copying in MMSE, overlapping figures, clock drawing test, cube copying, and line orientation. Four items in ICs (ICs-4) were found to be significantly impaired in DLB compared with AD, and a sensitivity and a specificity of total score of ICs-4 were 88.6% and 37.1%, respectively. When a score of ICs-4 is combined with a 10-point scaled score of pentagon copying in MMSE, a sensitivity and a specificity were 77.1% and 82.9%, respectively. The present study suggests that ICs-4 can be included in neuropsychological examinations to assess visuoperceptual impairment in DLB.