In patients with type 1 diabetes an association has been found between an insertion/deletion (I/D) polymorphism in the gene for angiotensin I converting enzyme and the presence of diabetic nephropathy. Our objective was (i) to assess this association in a large cohort of patients with type 1 diabetes and (ii) to examine whether this finding also applies to type 2 diabetes. We examined 247 patients with type 1 diabetes of more than 10 years duration (135 patients > or = 20 years): Nephropathy was present in 114 and absent in 133 patients. Furthermore we separately analyzed 455 patients with type 2 diabetes of more than 10 years duration (158 patients > or = 20 years). Nephropathy was present in 247 and absent in 208 patients. Nephropathy was defined by confirmed presence of albuminuria > 30 mg/day (or > 20 micrograms/min). The I/D polymorphism was analyzed with PCR technique and alleles were visualized on 2% agarose gels after ethidium staining. Allele frequencies in the overall diabetic population did not differ significantly from the normal population. Distribution of genotypes was not significantly different between type 1 patients with and without nephropathy (P = 0.377). Also, no significant difference in genotype distribution was found between type 2 diabetic patients with and without nephropathy (P = 0.948). We conclude that no significant association between I/D polymorphism and nephropathy was demonstrable in either type 1 or type 2 diabetes, despite considerable statistical power of the patient sample and adequate duration of diabetes for nephropathy to become manifest.