Purpose: Whether adjuvant therapy impairs cognitive function in women with breast cancer (BC) is unclear. We determined the effects of adjuvant therapy on cognitive function in women with early BC. Methods: We consecutively and prospectively enrolled women aged 40-69 years who had a positive radiographic finding from the mammography screening program at Stockholm South General Hospital. All women completed the Headminder Web-based neuropsychological battery Cognitive Stability Index (CSI) for response speed, processing speed, memory, and attention before diagnosis (T1), after surgery and before adjuvant treatment (T2), 6 months after start of adjuvant treatment (T3), and after another 3 months of follow-up (T4). Women with BC were divided into those receiving chemotherapy, hormone therapy, or no adjuvant medical therapy. Women without a diagnosis of BC served as healthy controls. Results: Of the 146 women enrolled, 77 had BC of whom 18 received chemotherapy; 45, hormone therapy, and 14, no adjuvant medical therapy; 69 were healthy controls. Memory scores for women with BC were significantly lower than those for controls over time, even after controlling for age and education. Memory and response speed scores were lower after chemotherapy than before (P less than 0.01 for both). Processing speed and attention improved significantly over time in all groups, a result consistent with a practice effect. Conclusion: Our results indicate subtle changes related to time course and treatment. Especially, that chemotherapy may impair memory and response speed in women with BC, consistent with those reported by BC survivors after adjuvant medical treatment. (C) 2011 Elsevier Ltd. All rights reserved.