Abstract Oxyhemoglobin dissociation curves (ODC) were performed on blood from newly diagnosed, nonketotic diabetics prior to and following initial insulin treatment and from ambulatory juvenile diabetics before and after their usual morning insulin. In 10 newly discovered diabetics the average P 50 at in vivo pH was normal prior to insulin (26.2 mm Hg), decreased to 24.5 mm Hg ( p < 0.005) on the day following the initial insulin administration, and was within normal limits (26.9 mm Hg) when the diabetes was finally well controlled and red cell 2,3-diphosphoglycerate (2,3-DPG) had risen to elevated levels. Oxygen affinity of hemoglobin was closely correlated with the content of red cell 2,3-DPG ( r = 0.61, p < 0.001) but was unrelated to the level of hemoglobin A lc. In 40 juvenile patients the average P 50 was also normal prior to insulin administration but was significantly lower 3–4 hr after they had received their usual insulin dose ( p < 0.001). The study indicates that insulin administration to diabetics with high blood glucose levels may lead to transient decreases in red cell 2,3-DPG and in oxygen-releasing capacity of the red blood cells.