We studied bowel function in relation to 123 breast cancer cases among 7,702 women from the US NHANES I Epidemiologic Follow-up Study. Results suggest a slight increased risk of breast cancer for both decreased frequency of bowel movements (relative risk = 1.5, 95% confidence interval = 0.8, 2.7) and firm stool consistency (RR = 1.8, 95% CI = 1.0, 3.2.) These observations are consistent with an hypothesized association between constipation and increased risk of breast cancer.