Eleven patients with chronic renal failure who were being treated with haemodialysis three times a week were monitored for a total of 34 haemodialysis sessions. Erythrocyte 2,3-biphosphoglycerate (2,3-BPG) concentration was analysed immediately before initiation of bicarbonate haemodialysis and 1 h afterwards. The 2,3-BPG concentration was expressed relative to the haemoglobin tetramer (Hb4) concentration as the 2,3-BPG/Hb4 ratio and compared with blood gas analyses and biochemical variables important for characterizing uraemia. During the first hour of haemodialysis the 2,3-BPG/Hb4 ratio decreased (p < 0.002), but the magnitude of the decrease did not significantly correlate with the 2,3-BPG/Hb4 ratio measured before haemodialysis (p=0.104). The decrease is most likely to be caused by the haemodialysis procedure itself. Mechanical stress on the erythrocytes is believed to cause the 2,3-BPG to escape; it is then removed by haemodialysis. Physiologically, an increase in 2,3-BPG would be expected to counteract the hypoxia which is frequently observed during haemodialysis. However, the present results show the opposite, a decrease in 2,3-BPG. No significant correlation was shown between the haemoglobin concentration and the 2,3-BPG/Hb4 ratio before dialysis (p=0.414). The pH showed a significant positive correlation with the 2,3-BPG/Hb4 ratio before dialysis, whereas the arterial pO2 and the 2,3-BPG/Hb4 ratio before dialysis were insignificantly negatively correlated. The concentrations of calcium, phosphate, creatinine, urea and albumin did not correlate significantly with the change in 2,3-BPG/Hb4-ratio after 1 h. The 2,3-BPG/Hb4 ratio (p=0.03) sampled just before dialysis correlated significantly and positively with the total weekly dosage of erythropoietin given to the patients.