OBJECTIVE Increased blood pressure variability (BPV) in end-stage renal disease (ESRD) patients is proved to be a risk factor for cardiovascular disease [Tozawa et al. 1999]. The effect of ultrafiltration (UF) on BPV in hemodialysis (HD) patients has not been reported in the literature. This study was undertaken to define the effect of a single UF on BPV in HD patients. METHODS Prior and after HD with UF, 24-hour ambulatory BP monitoring (ABPM) was applied to each patient and then diurnal and nocturnal BP and BPV parameters (both before and after UF) were compared and correlated with UF values. RESULTS Increase in BPV after single UF in all groups was statistically significant (p < 0.05). Only the daytime systolic (DS) BPV increase (median 42.4%) was in positive correlation with delta body weight (body wt) (median 3.07%) or UF amount (r = 0.649, p < 0.01). CONCLUSIONS Large volume depletions and sympathetic hyperreactivity could explain the increase in BPV. Increased interdialytic weight gain requires more UF and subsequently BPV, morbidity and mortality also increase. Thus, considerable efforts must be made to prevent great interdialytic weight gain in HD patients.