This study compares M-mode echocardiographic ratio indices (ERIs) in dogs with mitral regurgitation due to chronic valvular disease (CVD/MR) for their ability to discriminate and quantify associated echocardiographic changes. Weight-based M-mode ERIs were determined retrospectively for 2 groups of dogs, normal (n = 47) and CVD/MR (n = 176). The CVD/MR group was further subdivided into those with (CVD/MR B, n = 74) and without (CVD/MR A, n = 102) left-sided congestive heart failure (LCHF). ERIs were compared descriptively (summary statistics) and also by receiver-operator characteristic curve areas (ROCAs) for their ability to discriminate between groups. Univariate logistic regression (LR) models were used to estimate the odds ratio for each ERI as a predictor of the CVD/MR condition as well as to predict LCHF within the CVD/MR group. A novel ERI (wdeltaA), designed to express body size-normalized short-axis stroke area, was sensitive (90%) and specific (98%) to discriminate between normal and CVD/MR at a cutoff value of 1.8 standard deviations (wdeltaA = 2.1). Prediction of LCHF within the CVD/MR group was more problematic, but an ERI expressing left atrial size (wLA) showed 76% sensitivity and 81% specificity at a cutoff value of 5.2 standard deviations (wLA = 1.55). Ours was a descriptive study of echocardiographic patterns of pathology associated with CVD/ MR and determined which ERIs are sensitive and specific for CVD/MR and for LCHE ERIs normalize echocardiographic data for body size in dogs and are well-suited to quantitative descriptions and analysis.