Recently, McKeith et al. proposed criteria for the clinical diagnosis of dementia with Lewy bodies (DLB). In our study the clinical features of four patients with progressive dementia, visual hallucinations, delusions, and parkinsonism were compatible with those of DLB. To evaluate the neurological and psychiatric features, responses to medications, magnetic resonance imaging (MRI), and single-photon emission computed tomography (SPECT) in patients with DLB, we compared the above DLB patients with age- and gender-matched Parkinson's disease patients showing no signs of dementia. Our DLB patients presented with mild tremor, moderate rigidity and akinesia, severe constitutional dysfunction and recurrent visual hallucinations and delusions. These psychiatric symptoms became worse by anticholinergic agents and dopamine agonists and were difficult to control using neuroleptic medications. MRI revealed atrophy of the cerebrum to be more accentuated in the parietal region. SPECT demonstrated hypoperfusion in the parietal and occipital lobes. These findings suggest that parietal lobe dysfunction is a feature of DLB. It may, therefore, be concluded from this study that brain MRI and SPECT are useful in the clinical diagnosis of DLB, and that great caution should be taken when prescribing longacting dopamine agonists and neuroleptics to such patients.