The angiotensin I-converting enzyme gene is one of the major genes of the renin-angiotensin and kallikrein-kinin systems and is a candidate gene for several cardiovascular diseases for which a genetic predisposition has been established. Based on analysis of the level of the enzyme in plasma, a genetic polymorphism of its expression has been well characterized. Use of a DNA marker on the angiotensin I-converting enzyme gene has shown that the gene polymorphism is located within the angiotensin I-converting enzyme gene itself. This review summarizes the results of various studies of this polymorphism in cardiovascular diseases and the hypotheses that can be proposed to explain this effect.