Lewy bodies in Parkinson disease could be innocent bystanders or active agents responsible for neuronal death. Eighteen elderly patients with a Parkinson syndrome were studied prospectively and selected postmortem on the presence of Lewy bodies (14 cases with Parkinson disease, four with dementia with Lewy bodies). Information on disease duration was available in 17 cases. While akinesia and rigidity were linked with the neuronal loss, the percentages of Lewy body bearing neurons and of alpha-synuclein immunoreactive neurons in the substantia nigra were not correlated with the symptoms or the disease duration, and appeared stable, involving 3.6% of the neurons on average. Such stability indicated that, during the whole course of the disease, the destruction of the Lewy bodies was equal to their production. In the model that is proposed here, the Lewy bodies are eliminated when the neurons that bear them die. With the hypothesis that neuronal death is directly related to Lewy bodies, it is possible to estimate their life span, which was calculated to be 6.2 months (15.9 months for any type of alpha-synuclein inclusion).