Hirano bodies are bright eosinophilic intracytoplasmic inclusions which have a highly characteristic crystalloid fine structure. They occur preferentially in the neuronal processes of the CA1 area in Ammon's horn in a wide variety of conditions, especially amyotrophic lateral sclerosis and parkinsonism-dementia complex on Guam, Alzheimer's disease, Pick's disease and 'normal' elderly individuals. Hirano bodies contain epitopes of actin, actin-associated proteins, tau, middle molecular weight neurofilaments subunit and a C-terminal fragment of beta-amyloid precursor protein. In addition to the CA1 area of Ammon's horn, they have also been identified in many other areas of the nervous system in humans and various experimental animals. Although usually observed in neurons, Hirano bodies may also be present in other cell types. It is the consensus that Hirano bodies in the pyramidal layer of CA1 originate largely from an age-related alteration of the microfilamentous system.