We analyzed the clinical courses of 94 patients with treated primary hypertension and initially normal serum creatinine concentrations (≤133 μmol per liter [≤1.5 mg per deciliter]) who were followed for a mean (±SD) of 58±34 months (range, 12 to 174) to determine the frequency with which renal function deteriorated and the factors associated with deterioration. Fourteen patients (15 percent) had an increase in serum creatinine concentrations (≥35 μmol per liter [≥0.4 mg per deciliter]); in 16 percent of the 61 patients with apparently good control of blood pressure, the serum creatinine concentration rose 59±33 μmol per liter (0.67±0.38 mg per deciliter). Despite good control of diastolic blood pressure (≤90 mm Hg), black patients were twice as likely as white patients to have elevations in serum creatinine (23 percent vs. 11 percent). Stepwise discriminant function analysis showed that a significant rise in the serum creatinine concentration was most likely to occur in association with older age, black race, a higher number of missed office visits, and employment as a laborer. We conclude that although renal function was preserved in 85 percent of patients with treated hypertension, it may deteriorate in some patients despite good blood-pressure control. Our observations may partly explain why hypertension, particularly among black persons, remains a leading cause of renal disease in the United States. MORE than 60 million Americans between the ages of 25 and 74 are estimated to have hypertension1 and are at increased risk for myocardial infarction, stroke, and renal disease. During the past 25 years, antihypertensive agents that interrupt most of the known pathogenic mechanisms for hypertension have been developed and are widely available for use. During this same period the incidence of and the mortality due to myocardial infarction and stroke have been reduced.234 In contrast, the ability of antihypertensive therapy to prevent progressive deterioration of renal function resulting from primary (essential) hypertension is less certain. Large treatment surveys have… © 1989, Massachusetts Medical Society. All rights reserved.