Cystatin C, a protein inhibitor of lysosomal cysteine proteinases, was demonstrated by immunohistochemical techniques to be present in the birefringent amyloid deposits of the small arteries in the cerebrum, cerebellum, and leptomeninges of 10 Icelandic individuals with hereditary cerebral hemorrhage with amyloidosis. Specimens from other organs were investigated in one of the patients, and amyloid angiopathy characterized by an immunoreactivity of cystatin C was found in a submandibular lymph node. No immunoreactivity of amyloid fibril protein AA, kappa or lambda immunoglobulin light chain, or prealbumin was observed. Significantly low cerebrospinal fluid concentrations of cystatin C were found in all 9 investigated individuals with hereditary cerebral hemorrhage with amyloidosis. The concentrations of beta 2-microglobulin, albumin, and IgG in the cerebrospinal fluid were within normal limits. Isoelectric focusing showed that cystatin C from the cerebrospinal fluid of 9 patients with hereditary cerebral hemorrhage with amyloidosis had an isoelectric point identical to that of normal individuals. This investigation demonstrates that hereditary cerebral hemorrhage with amyloidosis may be diagnosed by two laboratory methods: immunohistochemical investigation of cystatin C in brain tissue specimens and quantitation of cystatin C in cerebrospinal fluid.