The recent description of a polymorphism in the gene for angiotensin-converting enzyme (ACE), with the D allele associated with greater plasma levels of ACE, allows us to perform studies of the relationship between this polymorphism and chronic renal diseases in which the renin-angiotensin system could be implicated. We examined 155 patients with autosomal dominant polycystic kidney disease (ADPKD) with linkage to the PKD1 locus. The ACE insertion/deletion (I/D) polymorphism was amplified with the previously published flanking primers, and the polymerase chain reaction product was separated, sized on a 2% agarose gel, and visualized by ultraviolet transillumination. The ACE genotype distributions were 11.6%, 63.8%, and 24.5% for II, ID, and DD, respectively. There were no significant differences among the three genotypes with respect to mean age, sex distribution, and prevalence of hypertension. The ACE genotype distribution in patients with end-stage renal failure at the time of data compilation was similar to that of the entire study population. In the subgroup of patients who received renal replacement therapy before the age of 50 years, we found a significant association between DD genotype and onset of end-stage renal disease (ESRD) before the age of 50 years compared with II and ID (P = 0.017). We calculated the estimated median renal survival time as 51 years for the II genotype, 53 years for the ID genotype, and 48 years for the DD genotype. There were statistically significant differences between DD and ID patients (P = 0.025). In conclusion, we found DD genotype implies a worse renal prognosis based on both the significantly lower median renal survival time and significantly greater percentage of patients who reach ESRD before the age of 50 years, without implying a greater prevalence of hypertension.