Background: The frequency of colorectal adenomas and carcinomas was investigated in a large cohort of women with breast cancer in comparison with matched controls, since data on the occurrence of second tumors in women with breast cancer is controversial. Design: In a cohort study, 188 consecutive women (median age 57 years) with primary breast cancer and 376 age-matched women who served as controls were examined by total colonoscopy. Breast cancer patients and controls were compared for the frequency of colorectal adenomas and carcinomas. Results: Women with breast cancer showed a higher risk of colorectal adenomas than controls (14.9 vs. 9.3%, p = 0.047, OR 1.7, 95% Cl 1.0-2.9). This increased prevalence resulted primarily from an increased prevalence in the age group 65-85 (31 vs. 10%, p = 0.004, OR 3.8, 95% Cl 1.6-9.3). Colorectal carcinomas were found infrequently in both groups (2 in each group). Women with breast cancer receiving anti-estrogen therapy showed a trend towards a lower risk of adenomas compared to women without anti-estrogen therapy (3.7 vs. 17.2%, p = 0.053, OR 0.16, 95% Cl 0.0-1.1). Conclusions: Women with breast cancer above the age of 65 years have an increased risk of colorectal adenomas compared to women without breast cancer. Women with a diagnosis of breast cancer should especially be encouraged to participate in colorectal cancer-screening programs which, in most countries, call for screening of all average-risk individuals over the age of 50 years.