Background and Methods: Hypertension is a common problem in hemodialysis (HD). However, its behavior during the interdialytic period is not completely known and is infrequently monitored in clinical practice. Thus, for better understanding of interdialytic blood pressure (BP), we analyzed the interdialytic blood pressure profile using 44-hour ambulatory blood pressure monitoring (ABPM) data in 71 unselected, stable HD patients. Results: There was an increase in BP during the interdialytic period (awake day 1: 135/84 ± 23/14 mm Hg; awake day 2: 140/86 ± 22/15 mm Hg, p < 0.05; sleep day 1: 130/77 ± 24/15 mm Hg; sleep day 2: 136/80 ± 24/15 mm Hg, p < 0.05). The correlation between the average 44-hour BP and interdialytic weight gain (IDWG) was not significant (r = –0.07 for systolic BP and r = –0.09 for diastolic BP). The number of non-dipper patients was high, 77% on interdialytic day 1 and 83% on interdialytic day 2 for systolic BP. Uncontrolled hypertension (average 44 h BP ≧135/85 mm Hg) was diagnosed in 58 (55%) patients. Patients with uncontrolled hypertension had higher pre- and posthemodialysis BP, higher BP on each interdialytic day and night, and higher night/day diastolic BP ratio on the second interdialytic day. These patients were also taking a greater number of vasoactive medications (1.5 vs. 0.6 in those with controlled BP, p = 0.001). There were no significant differences related to kt/V, hematocrit, or weekly erythropoietin dose between patients with controlled or uncontrolled BP. Hemodialysis shift assignment (morning or afternoon) did not impact on BP levels or diurnal profile. Conclusion: In HD patients, interdialytic BP is often poorly controlled, there is a progressive rise in BP, and a trend toward loss of nocturnal decline in BP as the interdialytic period progresses. Further research is needed to determine whether treatment directed to interdialytic BP changes can alter outcomes in HD patients.