The effectiveness of cardiomyoplasty on cardiac function is discussed, and the four mechanisms proposed to explain cardiomyoplasty effectiveness are reviewed. The first such mechanism, termed the squeezing effect, suggests that skeletal muscle wrapped around the heart squeezes the heart in the same way as cardiac massage, resulting in direct improvement in cardiac function. Hemodynamic improvement is rarely detectable, but significant subjective improvement is commonly seen clinically. The second mechanism, termed the sparing effect, suggests that even if cardiac performance remains unchanged after cardiomyoplasty, contraction of the wrapped lattisimus dorsi muscle causes an increase in the slope of the end-systolic pressure-volume relationship, and a reduction in left ventricular wall stress. Myocardial oxygen consumption is thereby reduced. The third mechanism, called the girdling effect, suggests that cardiomyoplasty may act like an elastic girdle around the heart to prevent enlargement of the failing heart. The fourth mechanism, called the collateral effect, suggests that, when applied to the ischemic heart, cardiomyoplasty increases collateral blood flow to the myocardium, thereby benefiting cardiac function. However, the existence of unknown mechanisms is suggested by two phenomena that cannot be explained by these four mechanisms alone. Cardiomyoplasty was introduced as a method of direct cardiac assistance. However, it now appears that the relatively passive role of cardiomyoplasty in oxygen consumption saving and ventricular enlargement prevention may be of great importance.