Angiotensin-converting enzyme (ACE) is a zinc metallopeptidase, with primary known functions of converting angiotensin I into the vasoactive and aldosterone-stimulating peptide angiotensin II and inactivating bradykinin. There is high variability among individuals in ACE concentrations, mainly due to the presence of a genetic polymorphism. The ACE gene has, in fact, insertion/deletion polymorphism in intron 16, consisting of a 287-base pair Alu repeat sequence, with three genotypes: insertion polymorphism, insertion/deletion polymorphism, and deletion polymorphism. The genetic effect accounts for 47% of the total variance of serum ACE. The determination of this polymorphism has allowed researchers to study the implications of the ACE gene in many case-control studies of cardiovascular disease, including myocardial infarction and hypertrophic and dilated cardiomyopathy. We review the current knowledge about the ACE gene polymorphism and its implications in heart failure secondary to ischemic or idiopathic dilated cardiomyopathy. Interpretation of the results of studies about the role of this polymorphism are controversial. The repetition of epidemio-genetic studies and the creation of adequate experimental studies will help to definitively establish the pathogenetic role of the permanent increase in ACE expression associated with the deletion polymorphism genotype.