Purpose Women with a mutation in BRCA1 or BRCA2 face a lifetime risk of breast cancer of approximately 80%. current users. For ladies with up to one 12 months of tamoxifen use the odds ratio for contralateral breast malignancy was 0.37 (95% CI: 0.20 – 0.69; p = 0.001) compared to women with no tamoxifen use. Among women with one to four years of tamoxifen use the odds ratio was 0.53 (95% CI: 0.32-0.87; p = 0.01). Among women with four or more years of tamoxifen use the odds ratio was 0.83 (95% CI: 0.44 -1.55; p = 0.55). Conclusion Short-term use of tamoxifen for chemoprevention in BRCA1 and BRCA2 mutation service providers may be as effective as a conventional five year course of treatment. Statistical Package. Results Tamoxifen use was reported by 14.8% of the bilateral cases and by 24.7% of the unilateral controls (p = 0.0001). The characteristics of the 411 cases of bilateral breast malignancy and 1093 cases of unilateral malignancy are offered in Table 1. The univariate odds ratio ARQ 621 for tamoxifen use and contralateral breast malignancy (both genes combined) was 0.52 (95% CI: 0.37 to 0.73) and the adjusted odds ratio (controlling for radiotherapy and chemotherapy) was 0.53 (95% CI: 0.37 to 0.75) (p = 0.003) (Table 2). Table 1 Comparison of bilateral and unilateral breast cancer cases Table 2 Association between tamoxifen use and contralateral breast malignancy by duration of use and by mutation The protective effect of tamoxifen was comparable for BRCA1 and BRCA2 mutation ARQ 621 service providers; among the BRCA1 cases and matched controls the univariate odds ratio was 0.58 (95% CI: 0.39 to 0.86) and among the BRCA2 mutation service providers the odds ratio was 0.36 (95% CI: 0.17 to 0.75). The adjusted odds ratio was 0.58 (95% CI: 0.39 to 0.85) for BRCA1 mutation carriers and was 0.39 (95% CI: 0.19 to 0.83) for BRCA2 mutation service providers. There was no pattern of increasing protection against contralateral breast cancer with increasing period of tamoxifen use – ARQ 621 in fact the strongest protective effect was observed for ladies with less than one year of use (Table 2). Among women with up to one 12 months of tamoxifen use (either mutation) the odds ratio for contralateral breast malignancy was 0.37 (95% CI: 0.20 to 0.67; p = 0.001); among women with one to four years of tamoxifen use the odds ratio was 0.53 (95% CI: 0.32 to 0.87; p = 0.01); among women with four or more years of tamoxifen use the odds ratio was 0.83 (95% 0.44 to 1 1.55; p = 0.55). We performed a separate analysis of short-term tamoxifen (one year or less) restricting cases to those women who experienced a contralateral breast cancer four or more years after the diagnosis of the initial malignancy (and their matched controls). In this analysis a minimum of two years experienced elapsed between the MET date of last tamoxifen use and the date of contralateral malignancy. On average in this subgroup the contralateral ARQ 621 malignancy was diagnosed 7.4 years after the initial diagnosis (range 4 to 26 years). The protective effect of short-term tamoxifen use persisted in this subgroup well after tamoxifen was terminated (Table 3). Table 3 Association between Tamoxifen and contralateral breast malignancy by Tamoxifen duration with four or more years elapsed from first malignancy to contralateral malignancy We excluded women who experienced undergone an oophorectomy but not those who experienced natural menopause. Comparable risk reductions were observed with tamoxifen among those women who were diagnosed with second primary malignancy when post-menopausal and when they were pre-menopausal (Table 4). Table 4 Association between Tamoxifen and contralateral breast malignancy by Tamoxifen duration by menopausal status at time of second main malignancy in the bilateral case. BRCA1 and BRCA2 combined Discussion We observed that the risk of contralateral breast cancer was reduced by approximately 50% in service providers of BRCA1 and BRCA2 mutations when tamoxifen was given as adjuvant treatment for the initial breast cancer. Adjusting for the effects of chemotherapy and radiotherapy did not materially alter this estimate. This association has been reported before [3-5] – what it novel in the present study is that the observed protective effect was equal to or greater for ladies who used tamoxifen for a short time (less than.