Background: In the absence of echocardiography, identification of cardiomegaly via thoracic radiography is a necessary criterion for classification of disease severity in dogs with myxomatous mitral valve disease (MMVD). Objective: Modified-vertebral left atrial size (M-VLAS) facilitates objective radiographic assessment of the left atrium (LA) in 2 dimensions and identifies LA enlargement more accurately than existing methods. Animals: Sixty-four dogs with various stages of MMVD and 6 control healthy dogs. Methods: Retrospective case-control study. Medical records were searched for dogs with varying severity of MMVD. Modified-vertebral left atrial size, vertebral left atrial size (VLAS), vertebral heart size (VHS), and radiographic left atrial dimension (RLAD) were measured from thoracic radiographs and compared with echocardiographically derived measurements.