Background: Goiter prevalence in school-age children is an indicator of the severity of iodine deficiency disorders (IDDs) in a population. In areas of mild-to-moderate IDDs, measurement of thyroid volume (Tvol) by ultrasound is preferable to palpation for grading goiter, but interpretation requires reference criteria from iodine-sufficient children. Objective: The study aim was to establish international reference values for Tvol by ultrasound in 6-12-y-old children that could be used to define goiter in the context of IDD monitoring. Design: Tvol was measured by ultrasound in 6-12-y-old children living in areas of long-term iodine sufficiency in North and South America, central Europe, the eastern Mediterranean, Africa, and the western Pacific. Measurements were made by 2 experienced examiners using validated techniques. Data were log transformed, used to calculate percentiles on the basis of the Gaussian distribution, and then transformed back to the linear scale. Age- and body surface area (BSA)-specific 97th percentiles for Tvol were calculated for boys and girls. Results: The sample included 3529 children evenly divided between boys and girls at each year (x̄ ± SD age: 9.3 ± 1.9 y). The range of median urinary iodine concentrations for the 6 study sites was 118-288 μg/L. There were significant differences in age- and BSA-adjusted mean Tvols between sites, which suggests that population-specific references in countries with long-standing iodine sufficiency may be more accurate than is a single international reference. However, overall differences in age- and BSA-adjusted Tvols between sites were modest relative to the population and measurement variability, which supports the use of a single, site-independent set of references. Conclusion: These new international reference values for Tvol by ultrasound can be used for goiter screening in the context of IDD monitoring. © 2004 American Society for Clinical Nutrition.