Amyloid is identified microscopically as an amorphous extracellular hyaline material that exhibits "apple-green" birefringence with Congo red stains. Amyloid is not a chemically distinct entity, and currently available molecular methods are capable of identifying over 20 amyloidogenic precursor proteins. Some of the more common diseases associated with amyloidosis include plasma cell dyscrasias, chronic inflammatory disorders, hereditary-familial mutations involving transthyretin, Alzheimer's disease, and so-called "senile" or age-related amyloidosis. The amyloid deposits in these various diseases may be isolated to a single organ such as the heart or brain, or the amyloidosis may be systemic. The senile types of cardiac amyloidosis can result from overproduction of atrial natriuretic factor or from accumulation of otherwise normal or wild-type transthyretin. We present the case of an 83-year-old hospitalized woman with known atrial fibrillation and previous pacemaker implantation who had cardiac arrest unresponsive to attempted resuscitation. Autopsy disclosed prominent amyloidosis involving the left atrium, and subsequent molecular studies identified the amyloidogenic material as alpha atrial natriuretic factor. Since the clinical management and genetic implications of the various diseases associated with amyloidosis are markedly different, we stress the importance of molecular classification whenever possible.