BACKGROUND: Since 1971, the general U.S. population has been monitored for dietary iodine sufficiency by urinary iodine (UI) measurements through the National Health and Nutrition Examination Survey (NHANES). This report presents the UI levels for the population participating in NHANES 2003-2004. It is the third assessment of the U.S. population since NHANES III (1988-1994), when the median UI level was observed to decrease from NHANES I (1971-1974). METHODS: In 2003-2004, a stratified, multistage, probability sample of approximately 5000 participants per year were selected to participate in NHANES Household interviews, and specimen collection were performed. UI level was measured by inductively coupled plasma mass spectrometry on a random subsample of 2526 participants aged 6 years and older. RESULTS: The median UI level for the general U.S. population in 2003-2004 was 160 microg/L (95% confidence interval [CI] 146-172), and 11.3 +/- 1.8% of the population had a UI level below 50 microg/L. Children had a higher UI level than adolescents and adults. Among all (pregnant and nonpregnant) women of reproductive age, the median UI level was 139 microg/L (95% CI 117-156), 15.1 +/- 3.2% women had a UI level <50 microg/L, and Non-Hispanic blacks in this group had a lower UI level than other racial/ethnic groups. CONCLUSIONS: These findings affirm the stabilization of the UI level and the adequate iodine nutrition in the general U.S. population since 2000. Future surveys designed to achieve UI levels representative of pregnant women can improve the estimate of iodine sufficiency in this population subgroup. Continued monitoring of the population for iodine sufficiency is warranted because of groups at risk for iodine deficiency disorders.