We report a 65-year-old man with amyloidotic polyneuropathy, who first suffered from heart failure at the age of 57, 3 years before the onset of neurological symptoms. He had no obvious family history. We analysed the transthyretin gene of the patient and 6 asymptomatic family members using polymerase chain reaction (PCR). The single amino acid substitution of a methionine for valine at position 30, which is a common mutation of Japanese type I FAP patients, was found from the patient and his sister of 47 years. Though Type I FAP patients often have cardiac conduction block, they rarely have signs of heart failure until the end stage of the disease. This is the first report of Type I FAP with severe myocardial involvement, in which TTR mutation at position 30 was confirmed. The result revealed the clinical variation of Type I FAP.