Cardiovascular abnormalities are associated with hydrops fetalis in 26% of cases reported in the modern era. These include not only structural abnormalities, but also cardiac arrhythmias, failure, tumors, myopathy, infection, inflammation, infarction, and arterial calcification. Cardiac structural abnormalities may be causative or seen only in association with hydrops fetalis. Structural lesions that result in right atrial pressure or volume overload seem to be most commonly associated with hydrops fetalis. Fetal cardiac tumors, cardiomyopathy, myocarditis, myocardial infarction, and arterial calcification probably result in hydrops fetalis by a similar mechanism. Fetal tachyarrhythmia has been shown to result in elevation of atrial pressure and atrial natriuretic peptide. Fetal tachyarrhythmias are the most treatable of cardiac causes of hydrops fetalis. Fetal bradyarrhythmias are less easily treatable and less certainly a causative mechanism of hydrops fetalis.