This study evaluated the predictors of atrial fibrillation (AF) and adverse clinical events in patients with rheumatic mitral stenosis (RMS) in sinus rhythm. The patients who diagnosed with RMS in sinus rhythm were evaluated retrospectively between March 2003 and June 2013. The primary outcome was the development of new-onset AF with annual event rates and the secondary outcome was the incidence of clinical events including development of new-onset AF, systemic embolism and all-cause death during follow-up. Among 293 patients, AF developed in 60 (20.5%) patients with average annual event rate of 3.5%/year during mean follow-up period of 68.2 ± 36.6 months (median 72 months). All cause death or systemic embolism occurred in 7.2% (21 patients; all cause death 9, embolism 12) with an average annual event rate of 2.1%. In the multivariate analysis, large left atrium (LA) dimension and severe mitral stenosis (≤1.5 cm(2)) were independent predictors of AF development [HR 1.06, 95% CI: 1.02-1.10; P = 0.001, HR 1.97, 95% CI: 1.06-4.14; P = 0.032] after adjustment for confounding factors. Patients with enlarged LA (≥47 mm) had an average annual AF development rate of 6.0%/year. In patients with RMS in sinus rhythm, annual AF development rate was 3.5%/year and increased according to LA size and mitral stenosis (MS) severity. Because of very high risk embolism, RMS with enlarged LA dimension need focused follow up for early detection of AF development and clinical events.