Descriptive epidemiological findings for 7,696 patients with newly diagnosed thyroid cancer reported to the Surveillance, Epidemiology, and End Results (SEER) program for the years 1973 through 1981 are summarized. The preponderance of this tumor in women and of the papillary histologic subtype are well documented. The data suggest that previously reported increases in the incidence of thyroid cancer among Whites levelled off in the late 1970s. Differences in the incidence of this cancer according to ethnic group were particularly striking. Compared with White men and women, Puerto Rico Hispanics and Blacks had significantly lower thyroid cancer rates (weighted rate ratios ranged from 0.48 to 0.65). New Mexico Hispanic men and Chinese, Japanese, Hawaiian and Filipino men and women had significantly higher rates (weighted rate ratios ranged from 1.56 to 3.17). Elevated thyroid cancer rates for residents of Hawaii, regardless of ethnic group, were also a noteworthy finding. Variations in thyroid cancer risk according to ethnic group and geographical residence may reflect socio-economic or local environmental influences, including the possibility of a carcinogenic agent in volcanic lava.