Exposures in southwestern Utah to radioactive fallout (1951 through 1962) from atmospheric nuclear detonations at the Nevada Test Site (NTS) were followed by smaller exposures (1962 through 1979) from venting of underground nuclear detonations. The cancer incidence in a 1951 cohort (4,125) of Mormon families in southwestern Utah near the NTS was compared with that of all Utah Mormons (1967 through 1975). There were 109 more cases of cancer than expected (288[observed]/179[expected]). Leukemia was most prominent early (1958 through 1966), with 19 cases, five times more than expected (3.6). The excess of leukemia persisted into the later period (1972 through 1980), with 12 cases observed, 3.4 expected. There was an increase in lymphoma. Excess cases of thyroid cancer appeared early and a notable excess appeared later (14/1.7). An excess of breast cancer was noted later (27/14). There were more cancers of the gastrointestinal tract than expected. There was an excess of melanoma (12/4.5), bone cancer (8/0.7), and brain tumors (9/3.9). A subgroup with history of acute fallout effects had a higher cancer incidence. That these cases can be associated with radiation exposures is supported by a comparison between groups of the ratio of cancers of more radiosensitive organs with all other types of cancer.