The electrocardiographic fibrillatory wave amplitude in 148 patients with atrial fibrillation was measured. To assess its diagnostic importance in relation to the left atrial size and underlying etiology, M-mode echocardiography was performed on 89 patients. The patients are classified according to the type of atrial fibrillation (paroxysmal or chronic), the f wave amplitude (coarse or fine), and the underlying etiology (rheumatic or nonrheumatic). The average fibrillatory f wave amplitude and echocardiographic left atrial size in chronic and rheumatic atrial fibrillation are significantly greater than in paroxysmal and chronic nonrheumatic patients. Furthermore, the fibrillatory f wave amplitude correlates strongly with the echocardiographic left atrial size.