Obesity has been investigated as a risk factor for various malignancies, including colon cancer. A case-control study was conducted on patients in three colonoscopy practices in New York City to determine possible risk factors for colorectal adenomatous polyps, a known precursor lesion for most cases of colorectal cancer. Among 301 case subjects with incident adenomatous polyps (174 men and 127 women) and 506 control subjects (223 men and 283 women), an increased risk was observed with increasing body mass index in women (odds ratio 2.1, 95% confidence interval 1.1–4.0; for highest versus lowest quartile, linear trend P =.02). A nonsignificant trend was observed for men. The increased risk seen in women is consistent with prior observations regarding reproductive hormonal and dietary risk factors for colorectal cancer. [J Natl Cancer Inst 83:359–361, 1991]