Some renal diseases (e.g. diabetic nephropathy and IgA glomerulonephritis) cluster within families, consistent with a strong genetic component for the development or progression of these diseases, or both. In this context it is attractive to examine the insertion/deletion polymorphism of the angiotensin I-converting enzyme. This polymorphism determines the concentration of angiotensin I-converting enzyme not only in serum, but also in tissues and thereby presumably the locally available concentration of angiotensin II. Several studies have examined whether this polymorphism is associated with the development of diabetic nephropathy, but most of these failed to show such an association. Studies in patients suffering from IgA glomerulonephritis or other renal diseases, including diabetic nephropathy, demonstrated that the insertion/ deletion polymorphism plays a role in the progression of renal diseases and in the response to treatment with angiotensin I-converting enzyme inhibitor.