The purpose of this study was to identify predictors of development of chronic nonrheumatic atrial fibrillation within one year of onset, thereby minimizing the risk of embolic complications and death. We retrospectively studied 137 patients with new-onset nonrheumatic atrial fibrillation. Chronic atrial fibrillation developed in 30 patients at the end of one year (chronic group). Atrial fibrillation remained paroxysmal in 107 patients (paroxysmal group). Clinical characteristics, electrocardiograms, and echocardiograms at the time of the onset of atrial fibrillation were compared in the two groups. Patients in the chronic group were significantly older than patients in the paroxysmal group (70.1±8.2 vs. 62.4±11.0 years, p<0.01) and had a significantly higher incidence of congestive heart failure (13% vs. 3%, p<0.05) and diabetes mellitus (37% vs. 19%, p<0.05). The chronic group also exhibited higher cardiothoracic ratio (52.0±5.7% vs. 47.6±5.0%, p<0.01), greater f-wave amplitude in lead V₁ (1.48±0.91 vs. 1.06±0.45mm, p<0.05), larger left atrial dimension (41.0±6.4 vs. 34.2±7.6mm, p<0.01), and lower left ventricular ejection fraction (71.4±5.6% vs. 75.5±8.2%, p<0.05). The presence of four or more of the following seven factors strongly predicted the development of chronic nonrheumatic atrial fibrillation within one year (88% to 100%): age _??_65 years, congestive heart failure, diabetes mellitus, cardiothoracic ratio_??_50%, f-wave amplitude _??_2.0mm, left atrial dimension_??_38mm, and ejection fraction _??_76%.