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.