Roux-en-Y gastric bypass is a gastric reduction duodenal switch with a combination of restrictive and malabsorptive procedures. It is the most common gastric bypass procedure performed in the United States. Malabsorption causing nutritional deficiencies does occur, yet a PubMed literature search (1955-2009) returned no reports of malabsorption of anticancer agents after gastric bypass. To our knowledge, this is the first report of three cases of malabsorption of the anticancer agent tamoxifen after this procedure. The first patient was a 58-year-old woman who underwent Roux-en-Y bypass for morbid obesity. Two years later, she developed estrogen receptor-positive ductal carcinoma in situ of the breast, underwent lumpectomy and irradiation, and tamoxifen was started. Two years after that, she presented with concerns of potential malabsorption of the drug. Her plasma tamoxifen level was 28 ng/ml, which was below the lower limit of the therapeutic range (77-274 ng/ml for 10-30-mg/day regimens). The second patient was a 51-year-old woman who sought medical advice on risk reduction for breast cancer after receiving a diagnosis of atypical ductal hyperplasia of the breast. She also had a history of morbid obesity and underwent Roux-en-Y bypass. Tamoxifen was started to reduce her risk of breast cancer; her plasma tamoxifen level was subtherapeutic at 14 ng/ml. The third patient was a 53-year-old woman with estrogen receptor-positive breast cancer who underwent lumpectomy and was prescribed anastrozole, an aromatase inhibitor. She also underwent Roux-en-Y bypass for morbid obesity. As she experienced adverse effects while receiving anastrozole, the drug was discontinued, and tamoxifen 20 mg/day was started. Her tamoxifen plasma level was 52 ng/ml. Therefore, her tamoxifen dosage was increased to 20 mg twice/day. Six weeks later, her tamoxifen level was 120 ng/ml (therapeutic range 95-520 ng/ml for the increased dosage). These three cases suggest that steady-state serum tamoxifen levels should be evaluated in patients who have undergone Roux-en-Y gastric bypass. Until adequate data suggest otherwise, parenteral anticancer alternatives should be considered when systemic therapy is needed in a patient with malabsorption.