Two families have been described previously with the features of an autosomal dominant familial cerebral amyloid angiopathy with nonneuritic plaque formation. The clinical features of the cases were dementia, spastic paralysis and ataxia. It has now been established that both families were descended from a common ancestor and the case histories of 26 affected individuals in 5 generations of this pedigree are reported. An autopsy study has been performed on a recent case. The findings are described and compared with the four previously published autopsy studies in this family, which is then discussed in the context of recent advances in the nosology of familial disorders in which cerebral amyloid angiopathy and other forms of cerebral amyloid deposition occur.