The aim of the study was to evaluate the effect of an insertion/deletion polymorphism of the angiotensin converting enzyme (ACE) gene on progression of diabetic nephropathy. We performed an observational follow-up study of 35 patients with insulin-dependent diabetes and diabetic nephropathy. Patients were investigated during captopril treatment for a median of seven (range three to nine) years. Eleven patients were homozygous for the deletion allele (DD) and 24 were hetero- or homozygous for the insertion allele (ID + II). The two groups had comparable glomerular filtration rate, albuminuria and blood pressure at baseline and captopril induced nearly the same mean reduction in blood pressure--to 103 (SD 5) mm Hg in the DD-group and 102 (8) mm Hg in the ID + II-group. The rate of decline in glomerular filtration rate was significantly steeper in the DD group than in the other group (mean 5.7 (SD 3.7) versus 2.6 (2.8) ml/min/year, p = 0.01). In conclusion, the deletion polymorphism in the ACE gene reduces the long term beneficial effect of ACE inhibition on the progression of diabetic nephropathy in patients with insulin dependent diabetes.