Objective: To determine the frequency of atrial fibrillation in rheumatic mitral stenosis and to correlate it with the size of the left atrium. Study Design: Case series study. Place and Duration: This study was conducted in Cardiology department of Jinnah Hospital and Lahore General Hospital, Lahore for One year, starting from January 2017 to January 2018. Methods: 100 consecutive patients with mitral stenosis, aged over 18 years, admitted to the cardiology department enrolled in this study.

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... tients with ischemic heart disease, thyrotoxicosis, hypertension, chest infections and previous cardiopulmonary interventions were excluded. All patients underwent thyroid function tests, cardiac enzymes and a complete blood test. Basic electrocardiogram, chest radiography and echocardiography were also performed. Other variables studied were the symptomatology. Results: In one hundred patients, atrial fibrillation was observed in 35 cases (35%). The frequency of atrial fibrillation increased with age, being maximum (75%) after 50 years. Similarly, there was a strong linear correlation between atrial fibrillation and left atrial dimension (r = 0.0001). Dyspnea was the most frequent symptom (78%), followed by palpitations (52%). Conclusions: Rheumatic mitral stenosis is one of the most frequent causes of atrial fibrillation and the enlargement of the left atrium is associated with an increase in the frequency of atrial fibrillation in patients with mitral stenosis. Key words: Atrial fibrillation, mitral stenosis, size of the left atrium.