The relationship between blood pressure (BP) and mortality in hemodialysis patients has remained controversial. Some studies suggested that a lower pre- or postdialysis BP was associated with excess mortality, while others showed poorer outcome in patients with uncontrolled hypertension. We conducted a multicenter prospective cohort study to evaluate the impact of hemodialysis-associated hypotension on mortality. We recruited 1244 patients (685 males; mean age, 60 +/- 13 years) who underwent hemodialysis in 28 units during the two-year study period beginning in December 1999. Pre-, intra-, and postdialysis BP, and BP upon standing soon after hemodialysis, were measured in all patients at entry. Logistic regression analysis was used to assess the effect on mortality of pre-, intra-, and postdialysis BP, a fall in BP during hemodialysis, and a fall in BP upon standing soon after hemodialysis. During the study period, 149 patients died. Logistic models identified the lowest intradialysis systolic blood pressure (SBP) and degree of fall in SBP upon standing soon after hemodialysis as significant factors affecting mortality, but not pre- or postdialysis SBP and diastolic BP. The adjusted odds ratio for death was 0.79 (95% CI 0.64-0.98) when the lowest intradialysis SBP was analyzed in increments of 20 mm Hg, and was 0.82 (95% CI 0.67-0.98) when the fall in SBP upon standing soon after hemodialysis was analyzed in increments of 10 mm Hg. These results suggest that intradialysis hypotension and orthostatic hypotension after hemodialysis are significant and independent factors affecting mortality in hemodialysis patients.