A significant negative correlation (P less than 0.0001) exists between the mean blood pressure (MBP) and the rate of decline in creatinine clearance with time in years (BCr) in 446 subjects in the Baltimore Longitudinal Study on Aging (BLSA) followed serially on five or more visits over a period of 8 or more years. Even when the 118 subjects with possible renal and/or urinary tract pathology (category 1) and 74 subjects treated with diuretic and/or antihypertensive agents (category 2) were not included, this relationship remained in the 254 "normal" (category 3) subjects. Since both MBP and negativity of the BCr tend to increase with age, multiple regression analyses using both MBP and age as independent variables were performed to determine their respective influences on BCr. Both MBP and age significantly influenced BCr. When those subjects with hypertension (mean MBP greater than 107 mm Hg) were not included, however, the inverse relationship between MBP and BCr is lost, suggesting that an accelerated loss of renal function is observed primarily because of the impact exerted by individuals with blood pressures in the hypertensive range.