Nitric oxide is too short-lived to measure in vivo, but its production can be estimated by measuring its stable oxidation products, nitrites and nitrates, in serum. Renal elimination of these ions has been demonstrated, but the effect of renal function on their concentrations in serum is currently unknown. We evaluated serum and urine nitrates + nitrites as serum nitrogen oxides (sNOx), nitrogen oxide (NOx) clearance, and creatinine clearance in 71 patients on the Intensive Therapy Unit. The correlation between sNOx and plasma creatinine was strong and highly significant (P <0.001). These results suggest that renal function has a significant effect on sNOx concentrations. Studies in which the sNOx concentration is used as an index of nitric oxide production can therefore be interpreted only if renal function has been taken into account.