Approximately 740,000 Ci of 131I were released into the atmosphere from the Hanford Nuclear Site in Washington State during 1944-1957. The Hanford Thyroid Disease Study (HTDS), conducted to determine if thyroid disease is increased among persons exposed as children to that 131I, also investigated whether thyroid ultrasound (US) abnormalities might be increased. The HTDS cohort (n = 5199) was selected from 1940-1946 births to mothers with usual residence in seven Washington counties. Of these, 4350 were located alive, 3447 attended HTDS clinics (1992-1997), and 3440 (1747 females) had evaluable clinical results and sufficient data to characterize their Hanford 131I exposures. US abnormalities were observed in 55.5% of women and 37.4% of men. Thyroid radiation doses from Hanford 131I, which could be estimated for 3191 evaluable participants, ranged from 0.0029 to 2823 mGy (mean, 174 mGy). Estimated dose was not significantly associated with the prevalence of any US abnormality (p = 0.21), US nodules with maximum dimension 5 mm or more (p = 0.64), or average number of US nodules per person (p = 0.80 for nodules with maximum dimension 5 mm or more). These results remained unchanged after accounting for factors that might confound or modify dose-response relationships and for uncertainty of the dose estimates. This study does not support the hypothesis that 131I exposure at Hanford's dose levels and dose rates during infancy and childhood increases the prevalence of adult thyroid US abnormalities.