The nucleus of the ansa peduncularis in the substantia innominata frequently contains degenerating neurons in patients with Huntington's disease, Alzheimer's disease, schizophrenia, and possibly other neurological and neuropsychiatric conditions. A large number of the degenerating cells are found only exceptionally in neurologically normal patients who are without mental symptoms, and the significance of the lesion may be related to quantitative factors, analogous to granulovacuolar degeneration of the hippocampus. The cells show massive distention with solvent-extractable lipid-pigment vacuolar droplet material that imparts a distinctive light and electron microscopic appearance.