Reports on a study seeking to contribute to a better understanding of cancer causation, particularly for work-related breast cancer. Found a statistically significant association of breast cancer risk with work in jobs classified as highly-exposed; 10 years of work in such jobs was estimated to increase breast cancer risk by 42 percent. For many specific sectors, with small numbers of women workers, there were too few people to show significant results, but for work in agriculture, metal-related manufacturing, automotive plastics, food canning operations, bars and casino work, statistically significant excesses were observed.