Isolated atrial amyloid (IAA), one of the most common members of the family of age-related ("senile") amyloids, seems to play a role in the pathogenesis of atrial fibrillation. Patterns of IAA deposition were histologically studied in the hearts of 100 elderly patients. The incidence (%) and severity (grade 0-3) of atrial IAA deposits increase with age, from 75% incidence and 0.50 average grade in patients aged 51-60 years, to 86% incidence and 1.49 average grade in those aged 81-90 years. Deposits are more pronounced in females (88% incidence, 1.45 average grade) than in males (68% incidence, 0.79 average grade). Left atrial deposits are more pronounced (78% incidence, 1.25 average grade) than right atrial deposits (67% incidence, 1.09 average grade). The distribution of IAA in the walls of the left atrium is uneven, with deposits being more pronounced in the anterior wall than in both the posterior wall and the left appendage. IAA deposits are heavier (1.34 average grade) in patients with chronic atrial fibrillation than in those with sinus rhythm (1.01 average grade); the difference, however, lacks statistical significance. Hypertension, diabetes mellitus, hypertrophy of the heart, coronary atherosclerosis, and dilatation of the atria show no significant relationship to the incidence or severity of atrial amyloidosis.