An important purpose of blood transfusion in patients with sickle cell disease is to improve arterial oxygen saturation (SaO2) and thereby reduce red cell sickling. To investigate the degree of improvement in SaO2 by blood transfusion, we determined the hemoglobin oxygen affinity, transcutaneous oxygen saturation (Tc-SO2), and pulse rate before and after automated partial exchange transfusion (erythrocytapheresis). In 13 patients with sickle cell disease who underwent 24 erythrocytapheresis procedures, the mean oxygen tension at half saturation (P50) was significantly reduced from 30.4 +/- 2.2 to 26.0 +/- 1.6 mm Hg (P< 0.01) immediately after exchange transfusion. Mean Tc-SO2 values increased from 96.2 +/- 2.8 to 98.5 +/- 2.1% (P< 0.01). Approximately 50% of the increase in Tc-SO2 after erythrocytapheresis could be explained by the increase in hemoglobin oxygen affinity. An increase in arterial oxygen pressure (PaO2) following erythrocytapheresis, suggested by the calculated PaO2 in this study, may explain some of the increase in Tc-SO2. We conclude that improvement in Tc-SO2 in patients with sickle cell disease resulted from changes in hemoglobin oxygen affinity as well as blood oxygen pressure following erythrocytapheresis.