The DD genotype of the ACE gene predisposes to faster diabetic nephropathy (DN) progression but its role in DN development is more controversial. We reported previously, in type 1 diabetic patients, an association between faster DN progression and the PC-1 gene Q121 variant, which associates with insulin resistance in non-diabetic subjects. We investigated here whether the combination of the ACE DD genotype and the PC-1 Q121 variant predicts the development and/or progression of DN in type 1 diabetic patients.Type 1 diabetic patients either with (n=159) or without (n=122) nephropathy were evaluated in a cross-sectional study. DN was defined as the presence of microalbuminuria or persistent proteinuria in a subject with more than 10-year duration of disease and concomitant diabetic retinopathy, and with no evidence of heart failure or other renal disease. Seventy-five (47 male/28 female) type 1 diabetic patients with nephropathy in whom retrospective information with repeated measurements of serum creatinine was available, were analysed in a longitudinal study.No association of the PC-1 Q121 variant and the ACE D/D genotype with DN development was observed. However, the ACE DD genotype and the PC-1 Q121 variant were associated, both independently (P=0.02 and P=0.025, respectively) or in combination (P=0.02), with a faster rate of glomerular filtration rate decline. An interaction (P=0.03) was observed between the two genes in increasing the individual patient's risk of being a fast progressor.Our data suggest that, in type 1 diabetic patients, the ACE and the PC-1 genes interact in increasing the individual risk of having a faster DN progression.