The main purpose of this study was to describe electrocardiographic (ECG) changes in canine babesiosis, and to relate these to clinical severity, outcome and cardiac pathology. Four groups of dogs with babesiosis were studied: mild to moderate anemia, severe anemia, concurrent autoagglutination and concurrent complications. Lead II ECG was recorded at admission for 1 minute in all dogs (121). A six lead ECG was recorded in 88 dogs. Full necropsy was performed on 16 dogs (5 died on arrival, 11 had ECG recording). The following ECG changes were recorded in relatively high prevalence: sinoatrial blocks or sinus arrest (7%), ventricular premature complexes (7%), low R-amplitude (23%), prominent Q (13%), axis deviations (40%), prolonged QRS (32%), ST depression and coving (28%), large T (42%), and notched R (28%). Differences between groups were minor. There was a significantly higher prevalence of sinus bradycardia and irregular rhythm in the non-survivors. Gross pathological changes were pericardial effusion and hemorrhages. Histological changes were hemorrhages, necrosis, inflammation and fibrin microthrombi. The only correlation between pathology and ECG was low R-amplitude and pericardial effusion. The ECG changes were similar to the pattern described for myocarditis and myocardial ischemia, and together with the histopathological findings indicated that the heart suffers from the same pathological processes described in other organs in canine babesiosis, namely inflammation and hypoxia. As the clinical application of the ECG changes found in this study was limited, cardiovascular assessment should be based on functional monitoring rather than ECG.