Patterns of long-term health risk reduction after levels of environmental hazards decline have been documented, but are still not precisely understood. Nuclear plant shutdowns which eliminate radioactive emissions and reduce toxins in the environment and food chain have been previously linked with significant short-term declines in local infant deaths and child cancer cases. The Rancho Seco nuclear power plant in Sacramento County, California USA ceased operating in June 1989; no other operating reactor exists within 200 miles of the site. We examine official California Cancer Registry data on cancer incidence for Sacramento County vs. the entire state, using the last two years of reactor operation (1988-1989) as a baseline; the Registry began in 1988. Temporal trends are given for Standard Incidence Ratios of all cancers combined, and by gender, race/ethnicity, common types of cancer, and child cancer. Since the late 1980s, cancer incidence in Sacramento County has declined for 28 of 31 categories (genders, races, types of cancer); of these, 14 declines are statistically significant and 2 others borderline significant. The estimated reduction in cancer cases in the county over a 20-year period is 4,319. Many factors can result in lower cancer incidence over two decades, but elimination of radioactive isotopes should be addressed as one of these potential factors in future reports.