The purpose of this study was to determine whether cognitive function changes over time in women with breast cancer who received adjuvant therapy as compared to women with breast cancer who received no adjuvant therapy. Three groups of women (n=46) were studied; groups 1 and 2 consisted of women with stage I or II breast cancer. Group 1 received chemotherapy and group 2 received chemotherapy plus tamoxifen. Group 3 consisted of women with ductal carcinoma in situ who received no chemotherapy or tamoxifen. Cognitive function was evaluated at three timepoints. Time 1 occurred after surgery and before chemotherapy initiation in groups 1 and 2. Time 1 for group 3 occurred post-surgery. Time 2 occurred within 1 week after the conclusion of chemotherapy for groups 1 and 2 and at a comparable time for group 3. Time 3 occurred 1 year after Time 2. Women who received chemotherapy plus tamoxifen exhibited deterioration on measures of visual memory and verbal working memory and reported more memory complaints. Women who received chemotherapy alone also exhibited deteriorations in verbal working memory. Conversely, cognitive function scores improved in women who received no therapy, indicating practice effects. In conclusion, adjuvant chemotherapy in women with breast cancer can be associated with deteriorations in memory and this may persist over time. The addition of tamoxifen may lead to more widespread memory deficits. Copyright © 2005 John Wiley & Sons, Ltd.