Hypertension is common in hemodialysis patients and increases cardiovascular morbidity and mortality. We determined the prevalence of inadequate control of hypertension in 489 patients receiving hemodialysis and identified factors associated with uncontrolled hypertension. We interviewed the patients and abstracted demographic and clinical information from a computerized database. The prevalence of uncontrolled hypertension (average predialysis blood pressure, > or =160/90 mm Hg) was 62%. Ninety-one percent of patients with uncontrolled hypertension were receiving submaximal antihypertensive drug therapy, and 59% withheld their medications before dialysis. Uncontrolled hypertensives had a greater interdialytic weight gain (3.8% v 3.5%, P = 0.07) and a greater excess weight gain (3.1 +/- 1.6 kg v 2.5 +/- 1.4 kg; P < 0.05) compared with controlled hypertensives. Patients with uncontrolled hypertension showed higher interdialytic weight gain (2.7 +/- 0.06 kg v 2.2 +/- 0.13 kg; P < 0.05), were more likely to be black (94% v 81%; P < 0.05), were more likely to have hypertension as the cause of their end-stage renal disease (ESRD) (42% v 24%; P < 0.05), and had been receiving hemodialysis for a shorter time (4.3 +/- 2 yr v 6.1 +/- 0.9 yr; P < 0.05) compared with normotensive patients. There was significant correlation between diastolic blood pressure and both interdialytic weight gain (r = 0.31, P < 0.05) and percent weight gain (r = 0.34, P < 0.05) in the hypertensive but not in the normotensive patients (r = -0.21). Interdialytic weight gain and hypertension as a cause of ESRD were independent predictors of predialysis systolic blood pressure. We conclude that hypertension is uncontrolled in most patients undergoing hemodialysis. Submaximal antihypertensive therapy, excessive interdialytic weight gain, and withholding antihypertensive medication before dialysis are correctable factors potentially contributing to uncontrolled hypertension.