Nitric oxide synthase (NOS) converts L-arginine to the intermediate NOHA, which is then converted to citrulline and nitric oxide. In patients with sickle cell disease with relatively normal renal function (serum creatinine < 220 μmol/L, n = 161), plasma arginine levels are lower than controls (n = 29), consistent with previously published effects of plasma arginase activity. Plasma levels of the NOS inhibitor ADMA are much higher than controls, and the downstream products of NOS activity are lower than controls, including both NOHA and citrulline. Horizontal bars indicate median values, and p value is calculated by the Mann-Whitney test.