Background: Children are at particular risk for selenium deficiency, which has potentially serious medical implications. Reliable age-specific reference values for serum selenium concentrations in children are sparse, but are essential for the identification of selenium deficiency and decisions regarding selenium supplementation.Methods: Using electrothermal atomic absorption spectrometry we analyzed serum selenium concentrations from 1010 apparently healthy children (age range, 1 day to IS years) and from 60 patients on a protein-restricted diet because of inborn errors of metabolism. Reference intervals were defined according to recommended guidelines.Results: Medians for serum selenium concentrations showed a statistically significant age dependency: a decrease from the age <1 month (0.64 mumol/L) to 4 months (0.44 mumol/L); an increase to 0.62 mumol/L in the 4-12 months age group; constant values in children between I and 5 years of age (0.90 mumol/L); and an additional slight increase to reach a plateau between 5 and IS years (0.99 mumol/L). Of 43 children older than 1 year and on a protein-restricted diet, 87% showed serum selenium concentrations below the 2.5 percentile.Conclusions: Because of nutritional changes, serum selenium concentrations are significantly higher in older children than in infants under 1 year of age. The application of age-adjusted reference values may provide more specific criteria for selenium supplementation. Long-term protein restriction in children is reflected by a failure to achieve higher serum selenium concentrations with increasing age.