OBJECTIVE The purpose of the study was to determine the likelihood of spontaneous conversion of recent onset (< 24 hr) paroxysmal atrial fibrillation (Af) to sinus rhythm and to define clinical and echocardiographic characteristics which may predict it. METHODS One hundred fifty-three consecutive adult patients admitted to the hospital with recent onset Af (< 24 hr) were studied. In each patient history, complete physical examination, 12-lead electrocardiogram, chest X-ray, routine hematological studies, serum electrolytes, troponin, thyroid function studies and a complete echocardiographic evaluation were performed. Patients hemodynamically unstable, with recent myocardial infarction, unstable angina, average ventricular rate > 150 beats/min, hyperthyroidism, congestive heart failure, left ventricular hypertrophy, valvular heart disease, and on antiarrhythmic drugs at the time of admission, were excluded. Patients were monitored without antiarrhythmic therapy for at least 24 hr from the onset of Af. RESULTS Spontaneous conversion to sinus rhythm occurred in 109 patients (71.2%); among patients with spontaneous conversion 73.4% converted in the first 12 hr. Age, gender, other clinical characteristics, left ventricular dimensions and performance did not separate patients with or without spontaneous conversion. Left atrial size was significantly greater in patients without compared to patients with spontaneous conversion (p < 0.03); likewise increased left atrial size (> 40 mm) was seen more often in patients without compared to patients with spontaneous conversion (45% vs 22%, p < 0.05). CONCLUSIONS Spontaneous conversion to sinus rhythm occurred in 71% of patients with recent onset (< 24 hr) Af. Left atrial size was the only predictor of spontaneous conversion in this highly selected group of patients.