Clinical and neuropathological examinations have shown that Alzheimer's disease is not a single entity. An early onset form with predominant parietal symptoms (type I) can be differentiated from a late onset form with mainly general cognitive symptoms and only mild parietal symptoms (type II). In CSF of patients with Alzheimer's disease type I there are signs of reduced blood-CSF barrier function and the metabolites of serotonin and dopamine are reduced in early stages, while in patients with type II there is often a diminished blood-CSF barrier function and in later stages there are reduced monoamine metabolites. Studies of membrane lipids of the brain have shown that myelin lipids are reduced in type II while gangliosides are markedly reduced in type I. Based on this finding a method has been developed for the assay of gangliosides in CSF. An increase of ganglioside GM1 seems to be a hallmark for Alzheimer's disease, type I.