We studied two patients from a Japanese family with carpal tunnel syndrome (CTS). The biopsy samples obtained during CTS surgical release revealed deposits of amyloid that stained with antihuman transthyretin (TTR) antiserum. Single-strand conformation polymorphism analysis and sequence analysis of polymerase chain reaction (PCR)-amplified exons of the proband's TTR gene revealed a point mutation resulting in a substitution of histidine for tyrosine at position 114. The mutation was confirmed by PCR-primer-induced restriction analysis. Our findings account for clinical heterogeneity of TTR-derived amyloidosis, and suggest the importance of substitution itself for deposits of amyloid in CTS.