Deposition of amyloid in the atria (isolated atrial/cardiac amyloid) is fairly common in the aging heart. It consists of amyloid fibrils, formed both by atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) and the precursor molecule of ANP, proANP. This study examines whether amyloidogenic determinants (short peptides/amyloid forming favoring regions) exist in the sequence of NT-proBNP, the N-terminal part of proBNP, and if these determinants form amyloid-like fibrils in vitro. We have predicted a possible amyloidogenic determinant in the sequence of the NT-proBNP, and we conclusively show, after its synthesis, that it forms amyloid-like fibrils in vitro, utilizing transmission electron microscopy, X-ray diffraction, attenuated total reflectance Fourier-transform infrared spectroscopy, and polarizing microscopy. Thus, for the first time, in this study, a possible biological role is attributed to a certain, specific part of this important cardiac prohormone/natriuretic peptide, which acts as an important biomarker indicative of heart failure. Its possible direct involvement in isolated cardiac amyloidosis, atrial fibrillation, and other types of cardiac amyloidoses is indicated and discussed. Since these cardiac hormones and their prohormones play key roles in cardiovascular homeostasis through natriuresis, diuresis, vasorelaxation, and inhibition of renin and aldosterone secretion (pathophysiology of hypertension and cardiovascular regulation), we also try to suggest these specific, short peptides as possible future structural targets of efforts toward inhibiting formation of natriuretic peptide(s) amyloid.