Oxygen transport by erythrocytes was studied in eight patients on maintenance hemodialysis before, during and after a 2-week stay at an altitude of 2000 m. Dialysis was continued at that altitude. In all tests, blood samples were collected one or two days following hemodialysis. Pre-altitude tests: The patients exhibited anemia (hemoglobin concentration, Hb = 97.4 +/- 17 g/l). Due to an elevated red cell 2,3-diphosphoglycerate concentration (2,3-DPG) and mild metabolic acidosis, elevated standard and in vivo P50 values (pO2 at 50% oxygen saturation of hemoglobin, sO2) were measured. Altitude: Upon ascent, arterial pO2 decreased from 82 +/- 4 torr to about 60 torr, sO2 was lowered by 5%. After 2 weeks sojourn, pO2 and sO2 increased towards normal values. In contrast to healthy subjects, dialysis patients developed respiratory alkalosis (blood pH: +0.074) upon ascent. This caused a significant shift to the left of the oxygen dissociation curve (ODC), indicated by lowered in vivo P50-values (P50,vv,-2 torr). Red cell 2,3-DPG, P50,st (P50 at a blood pH = 7.4 and pCO2 = 40 torr), hemoglobin concentration and hematocrit showed a high day-to-day variability and did not change because of the altitude exposure. We interpret the increase of the oxygen affinity of hemoglobin in patients with renal anemia as beneficial, as it favors oxygen loading of hemoglobin in the lung during exposure to a hypoxic environment.