Structurally abnormal transthyretin is a precursor protein of amyloid fibrils in type I familial amyloidotic polyneuropathy (FAP). This variant transthyretin has an amino acid substitution of methionine for valine at position 30. The purpose of this study was to clarify whether this variant transthyretin also circulates in the cerebrospinal fluid (CSF) of patients with type I FAP. CSF transthyretin of the patients was purified and its primary structure determined. Sequence determination indicated that transthyretin consisted of a mixture of normal and variant transthyretin. Variant transthyretin was present in the CSF of all 5 Japanese FAP patients studied. The CSF concentration of variant transthyretin was high (0.72 +/- 0.15 mg/dl, mean +/- S.D.), suggesting that variant transthyretin is synthesized in the choroid plexus. Variant transthyretin was not present in any of 20 controls. The CSF concentration of total transthyretin in FAP patients was 1.74 +/- 0.42 mg/dl, which was not significantly different from controls.