Blood volume (BV) change during hemodialysis is often monitored by packed cell volume (PCV). This assumes erythrocyte volume is constant. We tested this by dialyzing 5 patients for 2 hours against high (154 mmol/l), normal (140 mmol/l) and low (126 mmol/l) dialysate sodium concentrations. Erythrocyte water content, calculated from measured blood and plasma water contents, decreased with high and increased with low dialysate sodium concentrations. Erythrocyte volume, calculated from mean corpuscular hemoglobin concentration (MCHC) decreased 3.8% with high concentration dialysate and increased 2.5% when dialysate concentration was low. These changes correlated significantly (r = 0.80, p less than 0.01) with alterations in plasma sodium. Mean corpuscular volume (MCV), measured with a Coulter-S Plus Counter did not alter because of a methodological artefact. BV change can be calculated from PCV when plasma concentrations of osmotically active substances are changed only if allowance is made for altered erythrocyte volume.