To determine the potential effect of hypertension on early renal function deterioration, 30 patients (13 normotensive and 17 hypertensive) with autosomal dominant polycystic kidney disease and initially normal renal function were retrospectively analyzed. The decline in renal function was estimated by the slope of the linear regression of creatinine clearance versus time. Analysis was made in terms of standardized slope (slope divided by its standard deviation, i.e., measured in standard deviation units). In the hypertensive group the mean standardized slope was significantly higher than in the normotensive group (-10.89 and -4.98, respectively; p < 0.001). The average mean arterial pressure was significantly lower in the normotensive group with respect to the hypertensive one (95 and 109 mm Hg, respectively; p < 0.0001). There was a significant negative linear relationship between the average values of systolic, diastolic, and mean arterial pressures and standardized slopes. The best prediction equation for renal function deterioration was obtained using a multiple linear regression model in which independent variables were maximum and average diastolic pressures. Therefore, prior to renal insufficiency, a high arterial pressure had a significant contribution to renal function deterioration.