Fifteen cases of diffuse Lewy body disease were identified in a systematic survey of all 216 brains referred to this hospital from a single health district in a single year. These cases presented with Parkinson's disease (40%), cognitive impairment (40%) and both (20%). Quantitative neuropathological studies using anti-ubiquitin immunocytochemistry revealed that dementia severity was related to cortical Lewy body density. The prevalence of diffuse Lewy body disease may have been underestimated in the past because of the neuropathological difficulties in making the diagnosis. Firstly, cortical Lewy bodies have a subtle appearance and are easy to overlook. Secondly, senile plaques are a common feature of diffuse Lewy body disease and may lead the unwary to make an erroneous diagnosis of Alzheimer's disease or "plaque-only Alzheimer's disease'. Diffuse Lewy body disease is a common and important cause of parkinsonian dementia, including the dementia of Parkinson's disease itself.