Antihypertensive therapy reduces the rate at which glomerular filtration rate (GFR) declines (delta GFR) in diabetic nephropathy; however, the optimal blood pressure is unknown. The quantitative relationship between treated blood pressure and delta GFR was analyzed retrospectively in 59 patients with established diabetic nephropathy and treated hypertension using weighted univariate and weighted multivariate regression. The GFR was calculated using the Cockcroft and Gault formula. More rapid GFR loss correlated most strongly with higher diastolic blood pressures (r = 0.70; P < 0.0001); for each millimeter of mercury of diastolic blood pressure, the GFR decreased by 0.69 mL/min/yr. This relationship remained present if those individuals with diastolic pressures greater than 90 mm Hg were eliminated from the study (r = 0.50; P < 0.001). The correlation for systolic blood pressure was weaker (r = 0.30; P < 0.05) and explained completely by covariance between systolic and diastolic blood pressures. The correlation for mean blood pressure (r = 0.59; P < 0.0001) fell between the correlations for diastolic and systolic blood pressures. Proteinuria, serum albumin concentration, and serum cholesterol concentration also correlated with delta GFR. In multivariate analysis, neither these indices of disease severity nor the initial GFR explained the correlation between delta GFR and diastolic blood pressure. Age, sex, race, type of diabetes, and percentage of glycosylated hemoglobin did not correlate with delta GFR.