No therapy exists for the Lewy body diseases, which are an important cause of dementia. We regard Lewy body diseases and Parkinson disease as components of a spectrum of disorders having a degree of Alzheimer disease neuropathology. Immunologic features of Alzheimer disease and Parkinson disease suggest the involvement of similar phenomena in Lewy body diseases pathogenesis. Based on the efficacy of anti-inflammatory medication in arresting the progression of Alzheimer disease and the presence of common immunologic features in Alzheimer disease and Parkinson disease, a case is made for therapeutic intervention at the immune system level. Anti-inflammatory medications appear to be an appropriate therapeutic approach to Lewy body diseases.