Changes in colloid osmotic pressure (COP), osmolality, blood urea nitrogen, glucose, electrolytes and arterial blood gases were correlated with the symptoms that occurred during 46 dialyses in 10 stable, chronic hemodialysis patients who had symptoms attributable to the disequilibrium syndrome. The onset and severity of symptoms were consistently related to fluctuations in COP. Saline administration failed to relieve them, whereas the continuous infusion of salt-poor human serum albumin (1 g/kg) or mannitol (1 g/kg) successfully controlled the COP fluctuations and most of the symptomatology. The relationship of the COP fluctuations to existing theories on the pathogenesis of the disequilibrium syndrome were discussed.