INTRODUCTION The long-term health effects of external thyroid irradiation are known to include excess hypothyroidism, thyroid nodules, and thyroid cancer, and in this study we attempt to quantitate the relative risk of internal irradiation of the thyroid, for induction of thyroid cancer. The effects of external irradiation of child thyroids have been summarized in BEIR [II (1) and by the NCRP (2). Internal irradiation of the thyroid from a mixture of radionuclides has occurred in children as a result of accidental exposure to fallout from nuclear weapons testing. Legger numbers of persons having received diagnostic and therapeutic doses from I used in medical applications. Apart from the Marshallese, studies of internally irradiated human populations have not revealed an increased risk of thyroid malignancy (1,2). For example, studies of a group of children exposed to 90,000 person-rad in Utah, pave not revealed any excess thyroid cancer. The fallout in Utah contained I and was reported to deliver up to several hundred rad of absorbed dose to thyroids of children who were less than 10 years of age (1,2). There gre several studies which report no carcinogenic effect from large doses, 9f I (2). For example, Holm reported that persons irradiated with I, with doses ranging between 6000 and 10,000 rad, exhibited no statistically significant increase in thyroid cancer (2). Studies of the children in the Marshall Islands con- ducted since 1954, on the other hand, do show a statistically significant increase in thyroid cancer in these irradiated subjects. Since the Marshall Isiands' children were exposed simultaneously to external and internal irradiation, we have analyzed the data in an attempt to relate each type of exposure, internal versus external radiation, to the observed thyroid health effects. The mixture of radionuclides, coggributing.to internal dose in the Marshallese, included mogtly short-lived I and I, and only 10-20% of the thyroid dose came from I, thus the radiobiological considerations differ greatly in these various exposure circumstances. Estimates of thyroid~absorbed dose were recently reassessed for people exposed to fallout in the Marshall Islands (3). The accidental exposure of people on March 1, 1954, occurred as a result of nuclear weapons testing. Over the years, several estimates of thyroid-absorbed dose were made (4,5). The earliest estimate of thyroid dose was reported by Cronkite (4) who indi-~- cated a population-averaged thyroid dose. A 1962 study by James (5) listed the most probable thyroid dose to girls who were 3 co 4 years old at the time of exposure. Howsver, thg James dose estimate was flawed by, she incorrect association of I and I dose relative to the dose from I The most recent assessment of dose provided detailed information on the type of nuclides in fallout, the mode of intake, and the contributions from internal and external sources. The study of Lessard et al. (3) established greater absorbed dose to people based upon greater intake of the shorter-lived radiolodines. The thyroid dose ranged from several hundred to five thousand rad, and the highest doses were assigned to young people. The revised dose estimates accounted for the radioactivity from all iodine isotopes. Uncertainties with the dose estimates are associated with the amount of radioactivity measured in the urine of the exposed people, the intake of the short-lived radiotellurium and radioiodine isotopes and percent of thyroid uptake as as determined from a physiologic model, errors in estimating the exact amount of each radioiodine isotope, the dose rate and pattern of energy distribution from this radioiodine mixture, and the shape and thickness of the thyroid. Adams et al. (6) reported the medical status of the Marshallese accidentally exposed to fallout. Through March 1985 there were 35 adénomatous nodules, 5 adenomas, 9 papillary carcinomas, 1 atypical adenoma or follicular carcinoma, and 2 occult papillary carcinomas. A comparison group of equal 20