Cancer incidence and mortality rates may be used to test hypotheses regarding cancer etiology suggested by other scientific studies. In response to reports of an excess of left-sided breast cancer, our analysis of more than 250,000 cases from the Surveillance, Epidemiology, and End Results (SEER) program confirmed an overall 5% excess that occurred for all races and stages of disease, and increased with age for invasive disease. Ongoing analyses are investigating the relation between ovarian cancer mortality trends and parity and oral contraceptive use, which affect risk and have changed over time in the United States. An analysis of the potential effect of SV-40 contamination of poliovirus vaccine found that after more than 30 years of follow-up, no association was found with significantly increased rates of ependymomas and other brain cancers, osteosarcomas, or mesotheliomas in the United States.