Left atrial dimensions were measured using cross sectional echocardiography in 37 patients with mitral valve disease and 30 normal subjects of similar ages. The anteroposterior (AP), superior-inferior (SI), and medial-lateral (ML) left atrial dimensions were determined at the end of ventricular systole using parasternal long and short axis and apical four chamber views (for SIa and MLa). To assess the reliability of these measurements cross sectional echocardiographic and angiographic left atrial volumes were compared in 19 patients with mitral valve disease, giving an excellent correlation. A moderate correlation was found between the anteroposterior dimension of the left atrium obtained using M mode echocardiography and that obtained using the parasternal short axis and long axis projections. In normal subjects a good correlation was found between SI and ML dimensions, while a lower correlation was found between SI and AP, and ML and AP dimensions. The SI dimension was the major axis of the left atrium and AP dimension the minor axis. In patients with mitral valve disease a good correlation was found between SI and ML dimensions, while SI and ML dimensions had a low correlation with AP dimensions. The AP dimension was the minor axis of the left atrium, while the SI and ML dimensions were not significantly different. All left atrial dimensions were significantly greater in patients with mitral valve disease than in normal subjects. Of 30 patients with at least one dimension increased, all three dimensions were abnormal in 16, two dimensions were increased in 10, and only one dimension was increased in four. AP, SI, and ML dimensions were abnormal in 25, 20, and 27 patients, respectively. Cross sectional echocardiography may provide a reliable estimate of left atrial dimensions. In patients with mitral valve disease a thorough examination of the left atrium using multiple cross sectional views is necessary to detect asymmetric left atrial enlargement and to measure the degree of left atrial dilatation.