The pulmonary wedge pressure, pulmonary artery and right atrium pressure of 197 patients with dominant mitral stenosis were correlated with the age of the patients and the rhythm of the heart. A positive relation was found between atrial fibrillation and age, atrial fibrillation and right atrial pressure, atrial fibrillation and pulmonary artery pressure, age and right atrial pressure and pulmonary artery wedge and pulmonary artery pressures. A positive relation between right atrial pressure and pulmonary artery or pulmonary artery wedge pressures was found only in the cases with atrial fibrillation. No statistically significant relation was found between atrial fibrillation and pulmonary artery wedge pressure, age and pulmonary artery wedge pressure or age and pulmonary artery pressure. The data suggest the possibility of a causal relationship between right atrial pressure and atrial fibrillation.