The Modification of Diet in Renal Disease Study was a multicenter trial of the effect of protein restriction and strict blood pressure control on the progression rate of chronic renal failure of multiple causes. At the first baseline visit, 1,795 screened patients with renal disease had blood pressure measured, antihypertensive medications recorded, glomerular filtration rate (GFR) determined by 125I-iothalamate clearance, a nutritional assessment, and a 24-hour urine collection to determine sodium and potassium levels. A total of 1,494 patients in this cohort were classified as hypertensive (83%) and the remainder (301 patients) as nonhypertensive. Ninety-one percent of the hypertensive subjects were on treatment, 54% being controlled to a blood pressure of < or = 140/90 mm Hg. To better understand the factors that contribute to the development of hypertension in chronic renal disease, some determinants of the prevalence of hypertension in this cohort were investigated. Compared with normotensive subjects, hypertensive patients were older (51.2 +/- 12.7 years v 46.6 +/- 13.1 years [mean +/- SD]), had a higher body mass index (BMI; 27.5 +/- 4.7 kg/m2 v 25.4 +/- 4.2 kg/m2), and had a lower GFR (37.8 +/- 19.6 mL/min/1.73 m2 v 50.1 +/- 25 mL/min/1.73 m2). All these differences were significant (P < 0.01). The prevalence of hypertension was significantly higher for men than for women (86% v 80%; P = 0.001), and for blacks than for whites (93% v 81%; P < 0.001). The prevalence of hypertension was higher in subjects with glomerular disease than in those with tubulointerstitial disease (85% v 62.6%; P < 0.001). The prevalence of hypertension varied inversely with GFR (from 66% at a GFR of 83 mL/min/1.73 m2 to 95% at a GFR of 12 mL/min/1.73 m2). The prevalence of hypertension varied directly with BMI (from 70% with a BMI at the 10th percentile to 94% with a BMI at the 97th percentile). This relationship was independent of GFR. Multiple logistic regression analysis showed five predictors in decreasing order of significance as determined by chi-square values: GFR, 83.2; BMI, 36.7; black race, 19.9; increasing age, 14.5 (all P < 0.001); and male gender, 5.1 (P = 0.024). Salt intake was not a determinant of blood pressure status. These results confirm previous reports indicating that hypertension in renal disease is determined by the level of renal function. For the first time, three factors known to predict blood pressure levels in populations with normal renal function were also shown to be determinants of blood pressure in renal disease: BMI, black race, and age. In addition, the data suggest that hypertension is inadequately treated in more than half of patients with chronic renal disease in the United States.