(d) Histopathology Table 4 presents individual histopathologic diagnoses obtained on thyroid tissues resected from 63 Marshallese within the various study groups. Two patients required second operations: an exposed Rongelap woman (No. 15), who had recurrence of adenomatous nodules in residual thyroid tissue 10 years after initial resection, and a man exposed on Utirik (No. 2279) whose thyroid nodule was considered benign on frozen section but was found to contain papillary carcinoma on permanent sections. Four patients (No. 37, 40, 1556, 2261) had no distinct diagnostic lesion on pathologic examination. Their palpable thyroid irregularities appeared to have been secondary to normal anatomic variation and/or nonspecific focal fibrosis. The diagnostic categories in Table 4 are essentially those recommended by the World Health Organization (114) with minor modifications; the term, adenomatous nodule, was used to designate focal proliferative lesions that did not fulfill the usual criteria of true neoplasms. These were characterized by histologic changes typical of adenomatous (nodular) goiter (114,115) or nodular hyperplasia (116), but they usually presented as localized rather than diffuse alterations, possibly in part because of early detection. As noted in Table 4, most of these had histologic evidence of hyperplasia, such as hyperplastic papillae or solid areas of hypercellularity, accompanied by variable amounts of fibrosis or other evidence of regressive changes. The term, atypical adenoma or atypical adenomatous nodule, was applied to respective lesions that had more pronounced hypercellularity or variations in architectural patterns or cytologic characteristics but were devoid of clear histologic evidence of carcinoma, such as transcapsular infiltration or vascular invasion. Infiltrative tumors designated papillary carcinoma often contained significant follicular components and therefore might justifiably be termed mixed papillary-follicular carcinomas. There was a conspicuous lack of psamomma body formation. In several, there were relatively few papillations, but cyto- logic features of the neoplastic epithelium (e.g., overlapping, vesicular nuclei) were deemed sufficiently distinctive to merit the designation, papillary. Such mixed tumors, even with marked predominance of follicular elements, were classed as papillary carcinomas in this study since most authorities (41,43,44,116,118,121-123,125,128), but not all (148,153), consider them to behave in a clinically benign fashion of pure papillary carcinomas. Pure follicular carcinomas, which tend toward more aggressive clinical behavior than papillary tumors (41,44,115,116,118,121,125,153) have not been observed in the Marshallese, although some of the follicular lesions (e.g., Nos. 2221, 3006, 5059) exhibited focal, partial capsular infiltration. The potential clinical expression of such lesions, had they not been excised at early Stages, obviously remains speculative. The term, occult papillary carcinoma, was used for those minute papil- lary tumors found coincidentally with nonneoplastic nodular disease. These were equivalent to the W.H.O. classification of "non-encapsulated (occult) sclerosing carcinoma” (114) and corresponded to the “non-encapsulated scler- osing tumor" described by Hazard et al. (124,126) and “occult sclerosing carcinoma" of Klinck and Winship (127). These were never found as isolated, palpable lesions, but in each instance occurred with variable degrees of fibrosis in association with hyperplastic adenomatous nodules. Their totally benign clinical behavior, even in the presence of regional lymph node - 63-