Table 2. Composition of initial pediatric study groups. Exposed Rongelap Exposed Ailingnae Control Group M F M F M In utero 3 1 - - - 5-10 yr 10-18 yr Total 6 5 22 4 9 21 3 5 ] 13 3 25 Age on March 1, 1954 0-5 yx 8 7 2 1 9 F - 16 7 5 28 When children reached 18 years of age, they were transferred to corresponding adult study groups, so that technically all were adults by 1972. Nonetheless, increasing numbers of children in the island populations, even though not belonging to specific study groups, have been seen by the medical team, making continuing participation by pediatricians important to the field surveys. 2. Control Populations A group of 115 Marshallese, principally from the village of Rita on Majuro Atoll, was originally selected in 1954 as a control population, since they had background and living conditions similar to those on Rongelap and Utirik. They were matched as nearly as possible by age and sex with each indi- vidual in the Rongelap exposed group. Single blood samples from this group provided the primary basis for initial evaluation of hematologic alterations in the exposed Rongelapese. By 1957 attrition by emigration necessitated study of additional unexposed Marshallese to provide statistically valid comparisons. These were selected primarily from blood relatives of the Rongelapese who were away from the atoll at the time of the accident, but were returning to Rongelap with the exposed individuals at the time of repatriation in 1957. The size, age, and sex distribution of this group compared closely with that of the exposed Rongelapese, and its shared genetic heritage was considerably closer than that of the original control group from Majuro. Nonetheless, a number of these individuals were periodically lost to follow-up and were eventually replaced by addition of unexposed Marshallese from other island communities. During the past decade, an additional comparison population has been examined to evaluate the prevalence of thyroid abnormalities throughout the Marshall Islands. This population was comprised of unexposed Marshallese from Rongelap, Utirik, Likiep, and Wotje Atolls. Though some of these people have been exposed to low levels of residual radiation while residing part time on their home islands, this exposure is not considered to be a significant deterrent to their use since thyroid effects of such exposure, if any are possible, would likely be undetectable (see Appendix I}. However, none of the unexposed -10-