This pilot study examines whether hormone therapy for breast cancer affects cognition. Patients participating in a randomised trial of anastrozole, tamoxifen alone or combined (ATAC) (n=94) and a group of women without breast cancer (n=35) completed a battery of neuropsychological measures. Compared with the control group, the patients were impaired on a processing speed task (p=0.032) and on a measure of immediate verbal memory (p=0.026) after controlling for the use of hormone replacement therapy in both groups. Patient group performance was not significantly related to length of treatment or measures of psychological morbidity. The results showed specific impairments in processing speed and verbal memory in women receiving hormonal therapy for the treatment of breast cancer. Verbal memory may be especially sensitive to changes in oestrogen levels, a finding commonly reported in studies of hormone replacement therapy in healthy women. In view of the increased use of hormone therapies in an adjuvant and preventative setting their impact on cognitive functioning should be investigated more thoroughly. Copyright © 2003 John Wiley & Sons, Ltd.