OBJECTIVE We sought to determine the likelihood and predictors of spontaneous conversion to sinus rhythm of recent-onset atrial fibrillation (symptoms <72 h). BACKGROUND Although spontaneous conversion of recent-onset atrial fibrillation is common, the likelihood and clinical and echocardiographic predictors have not been fully defined. Such data would be important for management of patients in whom early cardioversion is desired: Cardioversion could be delayed in patients with a high likelihood of spontaneous conversion, and it could be expeditiously pursued if spontaneous conversion is unlikely. METHODS We screened 1,822 consecutive adults admitted to the hospital with atrial fibrillation and prospectively identified 356 patients (45% male, mean age +/- SD 68 +/- 16 years) with atrial fibrillation of <72-h duration. The occurrence of spontaneous conversion to sinus rhythm and clinical and echocardiographic data were identified through retrospective chart review. RESULTS Spontaneous conversion to sinus rhythm occurred in 68% of the study group (n = 242; 95% confidence interval [CI] 63% to 73%). Among patients with spontaneous conversion, the total duration of atrial fibrillation was <24 h in 159 (66%), 24 to 48 h in 42 (17%) and >48 h in 41 (17%) (p < 0.001). Logistic regression analysis of clinical data identified presentation <24 h from onset of symptoms as the only predictor of spontaneous conversion (odds ratio 1.8, 95% CI 1.4 to 2.4, p < 0.0001). Normal left ventricular systolic function was more common among patients with spontaneous conversion (p = 0.03), but it was not an independent predictor of conversion. Left atrial dimension was similar between groups. CONCLUSIONS Spontaneous conversion to sinus rhythm occurs in almost 70% of patients presenting with atrial fibrillation of <72-h duration. Presentation with symptoms of <24-h duration is the best predictor of spontaneous conversion.