OBJECTIVE: The use of tamoxifen to prevent breast cancer and decrease recurr en ce is not controversial. However, the effect that tamoxifen may have in women wi th a history of breast cancer in whom endometrial cancer develops is unclear. Th e purpose of this study was to estimate whether a history of tamoxifen use is a prognostic factor for such patients. METHODS: Between 1990 and 2002, patients se en at The University of Texas M. D. Anderson Cancer Center with a history of bre ast cancer who developed endometrial cancer were identified. Medical records wer e reviewed to identify clinical, pathologic, and outcome information. RESULTS: E ighty-nine patients with a history of breast cancer in whom endometrial carcino ma developed were identified. Fifty-two percent (46/89) had a history of tamoxi fen use (median duration 48 months; range 2-120 months). There were no signific ant differences in the clinical or pathologic features between tamoxifen users a nd nonusers. A history of tamoxifen use was associated with a shorter interval f rom breast cancer to endometrial cancer diagnosis (77.2 versus 121.3 months for nonusers; P = .01). There was no significant difference in overall survival betw een tamoxifen users and nonusers (39.2 months versus 48.3 months, P = .27), and there was no difference in endometrial cancer-specific survival duration betwee n tamoxifen users and nonusers (55.7 versus 51.0 months, P = .92). CONCLUSION: A mong tamoxifen users, the interval from breast cancer to endometrial cancer diag nosis was significantly shorter than that in nonusers. In this cohort, a history of tamoxifen use was not associated with a worse overall or disease-specific s urvival. OBJECTIVE: The use of tamoxifen to prevent breast cancer and decrease recurr en ce is not controversial. However, the effect that tamoxifen may have in women wi th a history of breast cancer in whom endometrial cancer develops is unclear. Th e purpose of this study was to evaluate whether a history of tamoxifen use is a prognostic factor for such patients. METHODS: Between 1990 and 2002, patients se en at the University of Texas MD Anderson Cancer Center with a history of bre ast cancer who developed endometrial cancer were identified. Fifty-two percent (46/89) had a history of There were no significant ant differences in the clinical or pathologic features between tamoxifen users a nd nonusers. A history of tamoxif en use was associated with a shorter interval f rom breast cancer to endometrial cancer diagnosis (77.2 versus 121.3 months for nonusers; P = .01). There was no significant difference in overall survival between tamoxifen users and nonusers (39.2 months versus 48.3 months , P = .27), and there was no difference in endometrial cancer-specific survival duration betwee n tamoxifen users and nonusers (55.7 versus 51.0 months, P = .92). CONCLUSION: Am tamoxifen users, the interval from breast cancer to Endometrial cancer diag nosis was significantly shorter than that in nonusers. In this cohort, a history of tamoxifen use was not associated with a worse overall or disease-specific s urvival.