The aim of the present study was to verify the validity of two-dimensional echocardiographic (2DE) measurement technique in dogs with left atrial enlargement. Thirty-one dogs with clinically and echocardiographically proven cardiac diseases were randomly selected. The left atrial right-to-left diameter (La r-l ) and the diameter of the left atrium just above the mitral annulus (La ama ) as well as the ratio of left atrial to aortic diameter (La/Ao) were measured by 2DE. The La r-l / La ama ratio was independent of body weight (R 2 = 0.0006) and age (R 2 = 0.0012), respectively. A significant linear relationship was found between La ama and La/Ao (the ratio that expresses the severity of atrial enlargement): La ama = 1.2238 La/Ao + 1.1608 (R 2 = 021; P < 0.01). There was also a positive linear relationship between La r-l and La/Ao: La r-l = 1.6876 La/Ao + 1.2648 (R 2 = 0.27; P < 0.005). In our dogs with cardiac disease, the La r-l /La ama ratio showed no significant relation to the La/Ao index (R 2 = 0.028). Thus, La r-l /La ama proved to be independent of the severity of atrial enlargement. Based on the strong relationship stated in the equation between La ama and La r-l [La r-l /La ama = 1.31159 (95% confidence interval)], La ama can also be used for the assessment of left atrial size even in dogs with cardiac disease. Key words: Left atrium, dog, atrial enlargement, echocardiographyThe study of left heart function and diseases by ultrasonography includes the measurement of left atrial dimensions. However, while measurement methods of the left ventricle can be considered well established in all regards also in *