The role of anaemia in the pathogenesis of abnormal cognitive function in haemodialysis patients is only partly understood. Several contemporary studies have assessed cognitive function in haemodialysis patients using neuropsychological and neurophysiological tests, comparing the results obtained over a wide range of haematocrits before and after treatment with epoetin. Of the neurophysiological tests, which measure electrical impulses from the brain in the presence or absence of specific stimuli, the cognitive event-related potential (ERP) has been particularly useful in evaluating this issue. Changes in amplitude (increase) and latency (decrease) of a specific wave form (P300) seen in the ERP correlate with improved cognitive function in haemodialysis patients following initiation of dialysis therapy. Similar effects are also evident when haematocrit is increased. Marsh et al. (Kidney Int 1991; 39: 155 163) examined the effect of increasing haematocrit with epoetin therapy on the P300 amplitude and latency, and noted that after increasing haematocrit to 23.7 36% over 12 months with epoetin, P300 amplitude increased (P<0.025), and several neuropsychological test scores were significantly improved. Grimm et al. (Kidney Int 1990; 38: 480 486) found a significant decrease in P300 latency and statistically nonsignificant improvements in P300 amplitude when haematocrit was increased to 22.7 30.6%. Improvement in the electroencephalogram (EEG) and P300 latency has also been reported by others, including Sagales et al. (Kidney Int 1993; 44: 1109-1115). The effect of complete reversal of anaemia (haematocrits 31.6 42.9%) on cognitive function was recently revealed by Pickett et al. (Am J Kidney Dis 1999; 33: 1122 1130), who found improvement in several cognitive ERP manoeuvres and a significant decrease in EEG slowing (P< 0.02) at higher haematocrits. Taken together, these various studies suggest that graded increments in haematocrit following epoetin therapy have a progressive effect on improving cognitive function in haemodialysis patients.