Background: Evaluation of left heart size helps determine disease severity in dogs with myxomatous mitral valve disease (MMVD). Hypothesis/Objectives: Determine the ability of radiographic vertebral heart size (VHS) and vertebral left atrial size (VLAS) to predict LHE ECHO in dogs with preclinical MMVD. Animals: Seventy client-owned dogs with MMVD and no historical or present clinical or radiographic evidence of congestive heart failure (CHF). Methods: Retrospective cross-sectional study of dogs with same-day echocardiography and thoracic radiography. Receiver-operating characteristic (ROC) curves were used to assess the ability of VHS, VLAS, and VHS + VLAS to discern dogs with and without LHE ECHO , and clinically relevant cutpoints for these radiographic measurements were selected. Results: The ability of VHS and VHS + VLAS to predict LHE ECHO was moderate (area under the curve [AUC] VHS = 0.851; 95% CI, 0.762-0.941; AUC VHS + VLAS = 0.865; 0.783-0.947), and performance of VLAS and VHS + VLAS was not different from that of VHS alone. A VHS cutpoint of >10.8 had sensitivity = 91.1% (76.3%-98.1%) and specificity = 69.4% (51.9%-83.7%) for predicting LHE ECHO. A cutpoint of >11.7 had sensitivity = 32.4% (17.4%-50.5%) and specificity = 97.2% (85.5%-99.9%) for predicting LHE ECHO. Thirty (43%) of the 70 dogs had a VHS value of 10.9 to 11.7. Conclusions and Clinical Importance: Vertebral heart size >11.7 identified dogs with LHE ECHO and VHS ≤ 10.8 excluded dogs with LHE ECHO. A large percentage of dogs had VHS values intermediate to these cutpoints.