Thoracic radiographs and electrocardiograms from 137 dogs with heartworm infection were independently evaluated and comparisons were made of (1) the presence or absence of right ventricular enlargement as assessed by these two diagnostic tests and (2) the presence or absence of right‐sided congestive heart failure relative to the degree of right ventricular enlargement and ECG criteria of RVH.Radiographic evidence of severe, but not moderate, right ventricular enlargement was associated with right‐sided congestive heart failure. Similarly the presence of three or more, but not fewer than three, ECG criteria of RVH were associated with right‐sided congestive heart failure. If either the thoracic radiographs or the ECG indicated that severe right ventricular enlargement was absent, right‐sided congestive heart failure was seldom present. The most accurate correlation of the radiographic evaluation of severe right ventricular enlargement was afforded by the presence of three or more ECG criteria of RVH. ECG criteria of RVH such as R/Sv4, + Tv10MEAx, and S11are highly specific tests; while SV2is a highly sensitive but not highly specific test. The ECG is a good confirmation test of the presence of severe right ventricular enlargement and is a good exclusion test for the absence of right‐sided congestive hear