Tamoxifen is being used successfully in breast cancer patients as adjuvant hormonal therapy. The aim of this retrospective cohort study is to evaluate the impact of tamoxifen on gallstone formation in postmenopausal breast cancer patients. A total of 3165 patients who were treated for invasive breast cancer between 1990 and 1997 were reviewed. The data were collected from four university hospitals in a population-based registry. Among these patients, 2462 were excluded from the study owing to improper follow-up and other reasons. Premenopausal patients were also excluded. Of the 703 patients included in the study, 457 had received adjuvant therapy including tamoxifen, and the other 246 had not. Gallstone formation was assessed by annual abdominal ultrasonography. The mean follow-up period was 4.6 years (range 1-7 years). There were no significant differences between the groups of breast cancer patients treated with or without tamoxifen regarding the age of the patients at the time of breast cancer diagnosis, the age at menopause, the duration between the onset of menopause and the time the breast cancer was diagnosed, the presence of diabetes, and the body mass index. At the end of 5 years the incidence of gallstone formation in tamoxifen-treated patients was 37.4%, whereas it was 2.0% in patients who did not receive tamoxifen ( p < 0.0001). The incidences of gallstones being detected in 171 tamoxifen-treated patients were 0.4%, 3.7%, 24.4%, 33.1%, and 37.4% cumulatively during the first, second, third, fourth, and fifth years, respectively. Hence adjuvant tamoxifen therapy leads to gallstone formation in postmenopausal breast cancer patients and is most apparent after 3 years of treatment.