We compared the clinical and neuropsychological pattern of dementia with Lewy bodies (DLB) to Alzheimer's disease (AD) and Parkinson's disease with dementia (PD‐d). Sixteen patients clinically diagnosed with DLB were compared with two groups of patients with PD‐d (n = 15) and AD (n = 16) matched for level of dementia. Isolated cognitive impairment was the most common form of presentation in AD (93.8%) and DLB (31.3%) groups, while parkinsonism was in 100% of PD‐d subjects. Psychoses associated with cognitive impairment at the beginning of the disease were more frequent in DLB patients (31.3%) than in AD (6.3%) and PD‐d (0%) groups. There were no significant differences in Unified Parkinson Disease Rating Scale motor‐subscale scores between DLB and PD‐d patients. DLB and PD‐d patients performed significantly worse on attentional functions and better on memory tests than AD. DLB patients also showed lower scores than AD subjects on visual memory, visuoperceptive, and visuoconstructive tests. No significant differences were found between PD‐d group and DLB subjects on any neuropsychological test. We were unable to find any differences in cognitive tasks between PD‐d and DLB subjects. Clinical features and neuropsychological deficiencies of DLB (attentional, visuoperceptive, and visuoconstructive deficits) and PD (attentional deficits) compared to AD (amnesic syndrome) can contribute to accurate identification of these entities and to the understanding of the neuropathological and neurochemical substrate underlying these diseases. © 2003 Movement Disorder Society