Parkinson's disease and dementia with Lewy bodies are two common presentations of a single, underlying disease process (Lewy body disease) which is thought to be related to dysregulation of the synaptic protein, alpha-synuclein. This article discusses the nature of the relations between Parkinson's disease and dementia with Lewy bodies, and what can be learned from them about the causes of dementia in patients with established Parkinson's disease. This is an area of clinical practice which is of increasing importance as greater numbers of ageing patients survive longer with good treatment of their motor symptoms. Precise use of terminology and a clear understanding of the biological substrates underlying symptom formation are particularly helpful to both clinicians and patients.