We report results from a population-based case-control study of colorectal cancer among Chinese women in western North America (NA) and the People's Republic of China (China). A common protocol was used to assess reproductive characteristics and hormone use of 395 Chinese women (189 from NA and 206 from China) with cancer of the colon or rectum and of 1112 age-matched Chinese controls (494 from NA and 618 from China). In NA, risks for cancers of both the colon and rectum were lower among parous compared to nulliparous women (odds ratio for colorectal cancer, 0.6, P = 0.08), but the trend in risk was not smooth with increasing number of livebirths. This association with parity was absent for both cancer sites in China. There were no consistent patterns in the relationships between other reproductive factors (including age at menarche, age at first livebirth, menopausal status) and risk of colon and rectal cancer on either continent.

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Supported by Public Health Service Grants CA-36503 and CA-44854 from the National Cancer Institute, National Institutes of Health, Department of Health and Human Services; by Health and Welfare Canada Grant 6610-1472-53; by British Columbia Health Care Research Foundation Grant 112 (84-1); and by the National Academy of Sciences of the People's Republic of China.

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