| Abstract
| - A community-based case-control study of the effect of reproductive factors on risk of large bowel cancer in Australia is described. The study involved 155 cases (99 colon cancer, 56 rectal cancer) and 311 controls who were interviewed with regard to pregnancies and their outcomes, lactation, menstrual history, and oral contraceptive (OC) use. Increasing parity was associated with a decreasing risk of colon cancer: para 0, relative risk (RR)=1; para 1-2, RR=0.9, 95% confidence interval (CI)=0.4-1.8; para ⩾3, RR=O.4, 95% CI=0.2-0.8; later age at first live birth (AFLB) was associated with increasing risk (AFLB ⩽21 yr, RR=1; 22-25 yr, RR=2.3, 95% CI=1.0-5.5; ⩾26 yr, RR=2.7, 95% CI=1.2-6.2). These effects were independent of each other. Parity appeared to exert its predominant effect on risk of cancer of the right colon. OC use was more common among controls than cases (RR=0.5; 95% CI=0.3-1 .2 for ever vs. never users) and showed a dose-response effect in multiple logistic analysis. The pattern of pOint-estimate RR for rectal cancer was largely congruent with those for colon cancer but was not significantly different from 1.0.
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