Two types of familial cerebral amyloid angiopathy or hereditary cerebral hemorrhage with amyloidosis (HCHWA) have been described: the Icelandic type (HCHWA-I), and the Dutch type (HCHWA-D). Both are autosomal-dominant forms of amyloidosis restricted to the small vasculature of the brain and clinically characterized by recurrent strokes leading to an early death. In spite of their clinico-pathological similarities, the amyloid fibrils are structurally different. In the case of HCHWA-I, the amyloid protein is a degradation product of Cystatin C variant (gamma trace), a normal serum protein and an inhibitor of cysteine proteases. The amyloid protein is the expression of a genetic aberration, since it has been demonstrated that a point mutation occurred in the Cystatin C gene. On the other hand the amyloid protein in HCHWA-D type has very recently been shown to be related to Alzheimer's disease (AD) and Down's syndrome (DS) β-protein. However, the complete sequence of HCHWA-D β-protein obtained from leptomeninges was three residues shorter (39 instead of 42) than that reported for the insoluble plaque amyloid of AD. The distinct enzymatic cleavage at the carboxyl end of the β protein is consistent with the concept that the amyloid fibrils derive from a larger precursor by specific and partial degradation. The difference may reflect a particular type of proteolysis that occurs in the vessel wall and not in the brain parenchyma. The characterization of the amyloid protein of HCHWA-D and the recent findings that cerebral amyloid deposits in sporadic cerebral amyloid angiopathy and asymptomatic age-related amyloidosis are antigenically and structurally related to the β-protein of AD and DS indicate that cerebral β-protein deposition comprises a spectrum of clinico-pathological entities exhibiting diverse etiology, patterns of amyloid distribution and clinical manifestations. It is our hypothesis that some aberrant post-translational modifications and/or genetic variants of amyloid precursor proteins can cause gross structural modifications. As a consequence they may be only partially degraded, resulting in abnormally cleaved peptides. These peptides may undergo self assembly and/or enzymatic cross-linking to form insoluble amyloid fibrils. The processing of the β-protein precursor in different cell types may account for the heterogeneous pattern of clinical manifestations and amyloid deposition in this group of disorders that we designate cerebral “β-amyloid diseases.”