Body size is associated with the risk of many diseases, including diabetes, heart disease, and some cancers. To evaluate the association of body size with large bowel cancer, height and weight measurements were ascertained by telephone interview from 779 Wisconsin (United States) women with newly reported diagnoses of carcinoma of the colon and rectum. Controls (n = 2,315) interviewed for this case-control study were selected randomly from Wisconsin driver's license files and Health Care Financing Administration files. The effects of weight and height were examined using multiple logistic regression to control for potential confounding variables. In this study, weight adjusted for height increased the risk of colon cancer (odds ratio [OR] for 72.57-148.33 kg cf 36.29-58.05 kg = 1.4, 95 percent confidence interval [CI] = 1.0-1.9) but did not increase the risk of rectal cancer. Height did not influence risk for cancer of either the colon or the rectum. Left-colon subsite analysis showed especially strong associations with current weight and with percent change in weight since age 18. These data suggest that a dose-response relationship exists between body size and risk of colon cancer in women; body size did not appear to influence risk of rectal cancer.