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Our objective was to evaluate l-arginine and nitric oxide metabolite (NOx) levels in children with sickle cell disease (SCD) at steady-state and during vaso-occlusive crisis (VOC). Because alterations in nitric oxide production may have an important role in the pathophysiology of SCD, our second aim was to determine if a relationship exists between these levels and vaso-occlusive crisis (VOC).Plasma l-arginine and serum NOx levels were examined in 36 patients with SCD with 39 episodes of VOC and 10 children with SCD at steady-state. Daily levels were obtained in children requiring hospitalization.Steady-state l-arginine levels were normal in children with SCD. l-arginine levels were low, however, in children with VOC (37.4 ± 2.7 vs. 53.6 ± 4.6 μmol/L;P = 0.008) but returned to baseline during hospitalization. In contrast, NOx levels were normal at presentation but decreased during hospitalization for both patients with VOC and patients with acute chest syndrome (ACS) (21.1 ± 2.0, 17.4 ± 2.4, and 12.3 ± 1.6 μmol/L, respectively;P < 0.05). In the patients with VOC who had ACS develop, l-arginine decreased to the lowest levels at the time of the ACS diagnosis, correlating with decreasing NOx levels.These data suggest that there may be a relationship between the l-arginine–nitric oxide pathway and vaso-occlusion in SCD. Low arginine levels during VOC could reflect a state of acute substrate depletion that results in a decrease in nitric oxide production.