The purpose of this study is to disclose the molecular basis of type I familial amyloidotic polyneuropathy (FAP) in Japan. Amyloid fibril protein of type I FAP consists of a variant transthyretin (also called prealbumin) with one amino acid substitution of methionine-for-valine at position 30. This variant transthyretin is present in the serum as a precursor protein of amyloid. A radioimmunoassay (RIA) has been established to detect the variant transthyretin. All the 94 patients with FAP who originate from various districts in Japan have the variant transthyretin, but any one of 78 healthy adults of families with FAP do not have it. Half of the symptom-free children of FAP patients have the variant transthyretin even before clinical manifestations appear. The RIA is widely applicable for early diagnosis. The methionine-for-valine substitution is due to a base change from guanine to adenine at the first letter of the valine codon at position 30. Type I FAP in Japan is considered to be a molecular disorder of transthyretin. Since the age of onset ranges from twenties to forties, genetic counseling is recommended to prevent the transmission of this intractable disorder to the next generation.