Combined dialysis and ultrafiltration leads to more frequent episodes of hypotension than isolated ultrafiltration. It has been suggested that decreased plasma volume preservation could be responsible for this phenomenon. The present study evaluates the effects of diffusive dialysis on the changes in relative blood volume (RBV). Six stable hemodialysis patients, without the need of ultrafiltration, were studied during 10 sessions of diffusive dialysis (bicarbonate) lasting 4 h. RBV was monitored continuously by measurement of hematocrit. During the 1st and 2nd h RBV increased by 2.4 ± 1.4 and 2.5 ± 0.8% respectively, returning to baseline levels at the end of dialysis. No changes in blood pressure or heart rate were noted. We conclude that during diffusive dialysis without ultrafiltration RBV is increased. A decrease in vascular resistance, or changes in regional blood distribution could explain these findings.