Myxomatous mitral valve disease (MMVD) is the most prevalent cardiac disease in dogs worldwide. Left atrial enlargement is a prominent feature associated with an increased risk of congestive heart failure in dogs with MMVD. Left atrial anteroposterior diameter (LAD) has been suggested as a supplementary parameter for evaluating LA size in various species. The purpose of our study was to establish a correlation between the left atrial anteroposterior diameter normalized to body weight (LADn) and N-terminal pro-brain natriuretic peptide (NT-proBNP) in dogs with MMVD and to assess the diagnostic potential of LADn for MMVD. We conducted this study on 35 dogs and categorized them into the control and MMVD groups. The control group consisted of 10 dogs, whereas the MMVD group included 25 dogs classified into stages B1, B2, and C according to the American College of Veterinary Internal Medicine (ACVIM) guidelines. LADn was measured using echocardiography, and NT-proBNP levels were assessed using a fluorescent immunoassay. In conclusion, LADn shows promise as a valuable diagnostic marker of MMVD in dogs. The strong positive correlation between LADn and NT-proBNP levels demonstrates the usefulness of LADn as a clinical index for evaluating cardiac function and disease progression in dogs with MMVD.