T he hallmark of sickle cell disease (SCD) is the polymerization of deoxygenated sickle hemoglobin (HbS). In SCD patients, one strategy to reduce red blood cell (RBC) sickling is to increase HbS oxygen affinity. Our objective was to determine if low concentrations of nitric oxide (NO) gas would augment the oxygen affinity of RBCs containing homozygous HbS (SS). Blood containing normal adult hemoglobin (AA) or SS RBCs was incubated in vitro in the presence of varying concentrations of NO up to 80 ppm, and oxygen dissociation curves (ODCs) were measured. In addition, blood was obtained from three AA and nine SS volunteers, before and after breathing 80 ppm NO in air for 45 min, and the ODCs were measured. Exposure of SS RBCs to 80 ppm NO in vitro for 5 min or longer decreased the partial pressure of oxygen at which hemoglobin is 50% saturated with oxygen (P50), an average of 15% (4.8+/-1.7 mmHg mean+/-SE; P < 0.001). The increase in SS RBC oxygen affinity correlated with the NO concentration. The P50 of AA RBCs was unchanged (P > 0.1) by 80 ppm NO. In SS volunteers breathing 80 ppm NO for 45 min, the P50 decreased (P < 0.001) by 4.6+/-2.0 mmHg. 60 min after NO breathing was discontinued, the RBC P50 remained decreased in five of seven volunteers in whom the ODC was measured. There was no RBC P50 change (P > 0.1) in AA volunteers breathing NO. Methemoglobin (Mhb) remained low in all subjects breathing NO (SS Mhb 1.4+/-0.5%), and there was no correlation (r = 0.02) between the reduction in P50 and the change in Mhb. Thus, low concentrations of NO augment the oxygen affinity of sickle erythrocytes in vitro and in vivo without significant Mhb production. These results suggest that low concentrations of NO gas may offer an attractive new therapeutic model for the treatment of SCD.