Oxygen transport was evaluated in patients with hemoglobin Köln. This hemoglobinopathy is characterized by an increased rate of destruction of red blood cells (RBC), a loss of heme groups from intracellular hemoglobin of surviving RBC, and an impairment of oxygen release from hemoglobin. Hemoglobin concentration, cardiac output, and oxygen consumption were essentially normal. Mixed venous oxygen tension was decreased, averaging 30 mm Hg. Renal blood flow was reduced by one third to one half. Evidence of latent hypoxia and decreased reserve were suggested by increased erythropoietin, decreased mean end capillary oxygen pressure, and evidence of blood flow redistribution. While essentially asymptomatic at rest and with ordinary activity, the subjects are probably more vulnerable than normal persons to diseases that would further impair oxygen delivery.