Despite a number of in vitro studies of transthyretin (TTR) amyloidogenesis the early stage of in vivo amyloidogenesis in the human heart is largely unknown. A heart with a mild degree of cardiac amyloidosis removed from a 90-year old woman at autopsy was selected for analysis. The genotype of the TTR was the wild type. An immunohistochemical study with anti-TTR antibody was performed on serial paraffin sections, and 17 TTR-positive lesions less than 50 micro m in diameter consisting of 13 interstitial and 4 vascular lesions were identified. The early interstitial lesions start as thick membranous deposits between interfacing myocardial cells. They are Congophilic with green birefringence and positive for apolipoprotein E but negative for amyloid P component. The TTR-positive amyloid extends along intercellular spaces and becomes larger, involving several myocardial fibers. The media is the initial site of arteriolar involvement. According to the in vitro studies of amyloid fibrillogenesis, the most critical step is formation of the nucleus under supersaturated conditions. The supersaturated conditions are speculated to be achieved by binding to proteoglycans or lipid membranes. Our results indicate that the basement membrane of myocardial cells is the initial site of amyloid deposition, providing a suitable place for concentration of TTR.