In order to define the relation between echocardiographically determined left atrial size and atrial fibrillation(AF) in rheumatic mitral valvular heart disease, 44 patients with mitral valvular heart disease with AF, 29 patients with mitral valvular heart disease without AF, 24 patients with idiopathic AF and 24 normal controls were studied. In mitral valvular heart disease with AF group, AF was more common when the absolute left atrial internal diameter(LAID) was above 50 mm(86.4%) than in mitral valvular heart disease without AF group(41.4%). In addition, although the values of LAID, LA(left atrium)/Aortic Root ratio and LA/BSA(body surface area) are higher in mitral valvular heart disease with AF group than in mitral valvular heart disease without AF group(p<0.05), there was no correlation between LAID and BSA(r=0.06). These findings suggest that an absolute value of LAID is a more important factor in the development of AF in mitral valvular heart disease than LAID considered in conjunction with BSA and that an absolute LAID of 50 mm may be used as a reliable guide to therapeutic and prophylactic intervention in AF associated with mitral valular heart disease.