This report presents urinary iodine (UI) concentrations for the general U.S. population during 2005-2006 and 2007-2008. These findings are the fourth and fifth assessments of the population since National Health and Nutrition Examination Survey (NHANES) III (1988-1994), when the median UI concentration for the population decreased from NHANES I (1971-1974).During 2005-2006 and 2007-2008, ~ 5000 participants per year were selected to participate in NHANES. The participants were interviewed and examined. UI concentration was measured on a random one third subsample of 2649 participants, aged 6 years and older in 2005-2006, and in all participants in 2007-2008. These urine iodine concentrations are representative of the general U.S. population by age, sex, and race/ethnicity.(i) The median UI concentrations for the general U.S. population in 2005-2006 and 2007-2008 were 164 mg/L (95% confidence interval [CI] 154-174) and 164 mg/L (95% CI 154-173), respectively. Also, the proportions of the population with a UI concentration of < 50 mg/L during these survey periods were 9.8% ± 1.3% and 8.8% ± 0.4%, respectively. The median UI concentration and prevalence of ≥ 200 mg/L appeared to be higher in children and persons ≥ 70 years than in other age groups. (ii) In both surveys, children aged 6-11 years had median UI concentrations of ≥ 200 mg/L, and about 5% of them had a UI concentration of < 50 mg/L. (iii) All pregnant women (sample size 184) surveyed during 2005-2008 had a median UI concentration of 125 mg/L (95% CI 86-198), and 56.9% ± 7.9% of this group had a UI concentration of < 150 mg/L. UI concentrations were lower among non-Hispanic black survey participants than non-Hispanic white and Mexican-American participants.These findings affirm the stabilization of UI concentration and adequate iodine nutrition in the general U.S. population since 2000. However, certain groups likely do not achieve a sufficient dietary iodine intake according to the World Health Organization. The needs of these vulnerable groups and the inadequacy of their dietary iodine intake should be addressed in future efforts.