Older literature suggested that the plasma sodium concentration is not individual - that it is neither intrinsic to an individual nor reproducible, longitudinally. We recently observed that plasma sodium concentration is heritable. Because demonstrable heritability requires individuality of the relevant phenotype, we hypothesized that plasma sodium concentration was substantially individual. In two large health plan-based cohorts, we demonstrated individuality of the plasma sodium concentration over a ten-year interval; intraclass correlation coefficient (ICC) averaged 0.4 - 0.5. Individuality of plasma sodium increased significantly with age. Plasma sodium individuality was equal to or only slightly less than that for plasma glucose, but was less than the individuality for creatinine. Individuality of plasma sodium was further confirmed by comparing the Pearson correlation coefficient for within-individual vs. between-individual pairs of sodium determinations, and via application of the agreement index. Furthermore, the distribution of all sodium determinations for all participants within a population was similar to the distribution for the mean sodium concentration for individuals within that population. Therefore, the near-normal distribution of plasma sodium measurements within a population is likely not attributable to assay-specific factors but rather to genuine and durable biological variability in osmotic set-point. In aggregate, these data strongly support the individuality of the plasma sodium concentration. They further indicate that serial plasma sodium values for any given individual tend to cluster around a patient-specific set-point, and that these set-points vary among individuals.