BACKGROUND The influence of interdialysis (ID) volume expansion on the blood pressure (BP) change and on the BP level at the end of the ID time period was studied in 167 chronic haemodialysis patients. Our analysis focused on 120 patients not receiving antihypertensive drugs (untreated group). The remaining 47 patients were receiving antihypertensive medication (treated group). METHODS The ID weight gain was considered equivalent to the volume gain. In each patient the mean ID BP change (as percent change of initial BP) and the mean ID volume expansion related to the lean body mass (ml.kg-1) were determined from 25 consecutive ID time periods. The individual volume sensitivity of BP was expressed as the BP change divided by the volume expansion. Basal overhydration was estimated as mean ID initial weight minus dry weight. RESULTS All patients gained volume during ID time periods and the BP was increased in 91%. The change of mean BP (MBP) was directly correlated with volume expansion (r = 0.45, P < 0.00001) only in the untreated group. These patients showed a volume sensitivity unrelated with age, serum urea and calcium concentrations and haematocrit. Sensitivity of diastolic BP (DBP), an indicator of the capacity to respond to volume expansion by vasoconstriction (autoregulatory process), exhibited a negative correlation with the initial DBP level (r = -0.36, P < 0.0001) and with the serum potassium (in women, r = -0.35, p < 0.02). These factors appeared to counteract the volume-induced DBP response. The MBP levels at the end of ID time periods were independent of volume expansion and basal overhydration. Hypertensive patients showed a higher sensitivity than normotensive patients (0.35 +/- 0.2 versus 0.20 +/- 0.19% per ml.kg-1, P < 0.005). Final MBP showed a positive correlation with initial MBP and, to a smaller extent, with serum urea concentration. CONCLUSIONS In our study the ID change of BP is partially dependent on volume gain. Volume sensitivity is a measure of the BP responsiveness and is higher in hypertensive patients. Final BP depends on the height of initial BP and other factors accounting for volume sensitivity, whose precise nature remain to be clarified.