Background We studied all patients admitted to hospital with first onset atrial fibrillation (AF) to determine the probability of spontaneous conversion to sinus rhythm and to identify factors predictive of such a conversion. Methods and Results We retrospectively reviewed charts of 438 consecutive patients admitted to hospital with first onset AF from January 1, 2006, to December 31, 2009. The patients were divided into two groups, recent onset AF defined as AF <48 hours or longer lasting AF, defined as AF > 48 hours. Spontaneous conversion occurred in 54%, (n = 203; 95% CI: 49 - 59%). In the group with first onset AF <48 hours, spontaneous conversion occurred in 77%, compared to 36 % in the group with first onset AF > 48 hours. Logistic regression analysis identified duration of AF as a highly significant predictor of spontaneous conversion to sinus rhythm (OR 5.9; 95% CI; 4.0 - 8.6, P <0.001). Conclusions Spontaneous conversion occurred in 54%, increasing to 77% when AF had persisted less than 48 hours.