ALZHEIMER'S disease, a fatal neurodegenerative disorder of unknown aetiology, is usually considered to be a single disorder because of tbe general uniformity of the disease phenotype1,2. Two recent genetic linkage studies revealed co-segregation of familial Alzheimer disease with the D21S1/S11 and D21S16 loci on chromosome 21 (refs 3,4). But two other studies, one of pre-dominantly multiplex kindreds with a late age-of-onset5, the other of a cadre of kindreds with a unique Volga German ethnic origin6, found absence of linkage at least to D21S1/S11. So far it has not been possible to discern whether these conflicting reports reflect aetiological heterogeneity, differences in methods of pedigree selection, effects of confounding variables in the analysis (for example, diagnostic errors, assortative matings), or true non-replication. To resolve this issue, we have now examined the inheritance of five polymorphic DNA markers from the proximal long arm of chromosome 21 in a large unselected series of pedi-grees with familial Alzheimer's disease. Our data suggest that Alzheimer's disease is not a single entity, but rather results from genetic defects on chromosome 21 and from other genetic or nongenetic factors.