Fifteen cases of diffuse Lewy body disease were diagnosed on pathological grounds during a single year in one health district. The range and frequency of clinical features contrast strikingly with previous reports. The majority of cases presented with classical levodopa-responsive Parkinson's disease either alone (6 cases) or with mild cognitive impairment (3 cases); the remaining 6 cases presented with cognitive impairment alone. In time almost all patients developed both dementia and Parkinsonism. The dementia was cortical in type, but unusual in that most (12 cases) showed day-to-day fluctuation in severity at some point in their illness. These findings suggest that diffuse Lewy body disease is not rare, and that it presents in a range of ways from dementia with subsequent Parkinsonism to Parkinson's disease with subsequent dementia. The latter mode of presentation suggests that it should be considered as a significant pathological substrate of dementia in Parkinson's disease.