Atrial fibrillation (AF) is frequently found in association with rheumatic mitral valve disease. To study the interrelation of factors contributing to the risk of AF in patients with mitral stenosis, we examined a cardiac catheterization database of a series of 314 patients. Patients with AF were older, 53.4 +/- 6.1 years versus 51.7 +/- 7.2 years (p = 0.03), and had a lower cardiac index, 2.3 +/- 0.6 L/min/m2 versus 2.6 +/- 0.7 L/min/m2 (p = 0.0002), than patients in sinus rhythm at catheterization. The mitral valve area was significantly smaller in patients with AF than in patients in sinus rhythm, 1.2 +/- 0.5 cm2 versus 1.6 +/- 0.7 cm2 (odds ratio 1.40/0.25 cm2 decrease; p = 0.0001) as was mitral valve index. The pressure-AF association with the highest statistical significance was seen with mean right atrial pressure, 10.6 +/- 4.9 mm Hg versus 7.6 +/- 3.8 mm Hg (odds ratio 2.24; p < 0.0001). Other variables with significant positive associations by univariate analysis were pulmonary artery wedge pressure, pulmonary artery mean pressure, and pulmonary resistance. When stepwise logistic multiple regression analysis was performed, the results indicated that both severity of mitral stenosis and increased right atrial pressure were independently associated with AF in this population with mitral stenosis. After adjustment was performed for these variables, age was not independently associated with AF.