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We have used data from a large population-based case-control study in the United States to evaluate the effect of occupational physical activity on breast cancer risk. Women diagnosed with breast cancer identified from four state cancer registries, and controls randomly selected from lists of licensed drivers or Medicare beneficiaries, were interviewed by telephone for information on usual occupation and other factors. We classified usual occupation into one of four categories of physical activity. After excluding subjects for whom a strength rating could not be assigned, we had a final sample size of 4,863 cases and 6,783 controls. Using conditional logistic regression models, we calculated adjusted odds ratios (OR) and 95 percent confidence intervals (CI) for occupations having light, medium, and heavy activity compared with sedentary ones. Women with heavy-activity occupations had a lower risk of breast cancer than women with sedentary jobs (OR = 0.82, CI = 0.63-1.08), as did women with jobs with medium activity (OR = 0.86, CI = 0.77-0.97) or light activity (OR = 0.92, CI = 0.84-1.01). There was a significant decreasing trend in the ORs from sedentary to heavy work (P = 0.007). Although limited by exposure misclassification, these data are consistent with the hypothesis that physical activity reduces the risk of breast cancer.

Data from a population-based case control study were used to estimate occupation-specific relative risks for female breast cancer, adjusted for established breast cancer risk factors. Breast cancer cases under age 75 were identified from tumor registries in four states. Controls were randomly selected from driver's license and Medicare beneficiary lists. Information on usual occupation and risk factors was obtained by telephone interview. Odds ratios from logistic regression adjusted for age, state, body mass index, benign breast disease, family history of breast cancer, menopausal status, age at menarche, parity, age of first birth, lactation history, education, and alcohol consumption were calculated for each of 26 occupational groups. Complete occupational information was obtained for 6,835 cases and 9,453 controls. Of 26 occupational groups, only "administrative support occupations" had a statistically significantly increased risk of breast cancer (OR = 1.15, 95% CI 1.06-1.24). In these data, no specific occupational group had an unusual risk of breast cancer. Increased risks reported elsewhere for nurses and teachers were not corroborated.