Haemodialysis is frequently complicated by side-effects both during and after treatment. Hypotension and muscle cramps have been attributed to depletion of intravascular volume; headache and fatigue have been attributed to rapid changes in intracellular and extracellular osmolality. An evidence-based systematic review of the English language literature was used to evaluate these hypotheses. Four studies have addressed the morbidity associated with changes in intravascular volume. These data suggest that during haemodialysis the vascular space is refilled from the interstitial space. Overhydrated patients are less likely to experience hypotension than are dehydrated patients, perhaps at the risk of congestive heart failure and hypertension. Intradialytic changes in haematocrit reflect changes in vascular volume and may be used to predict intradialytic hypotension. Eight studies have addressed the morbidity associated with changes in osmolality. In two of these studies the investigators reported clinical benefit for patients with patient-specific sodium profiles during dialysis. Four studies lacked sufficient statistical power to detect an effect of sodium profiling on patient symptoms. Two studies suggest a clinically important decrease in intradialytic symptoms during treatment with sodium-profiled dialysate. A definitive test of these hypotheses will require a randomized, blinded study of the clinical impact of sodium/ultrafiltration modelling on patient symptoms.