The authors estimate tamoxifen's impact on life expectancy among healthy women. A Markov model compared the effects of 5 years of tamoxifen on survival among 50-year-old postmenopausal women. Scenarios were explored using alternative assumptions with regard to tamoxifen's long-term effects on breast and endometrial cancer. Postmenopausal women without a uterus had substantial life expectancy gains from tamoxifen (1 to 4 months), whereas women with a uterus had such gains only if they were at a very high breast cancer risk. If tamoxifen's impact on endometrial cancer persists after treatment is discontinued, women at high risk for endometrial cancer have life expectancy losses from tamoxifen unless they are at a very high risk for breast cancer. The authors conclude that tamoxifen use among postmenopausal women is associated with substantial life expectancy gains. However, this benefit is modulated in women at increased endometrial cancer risk and depends on assumptions concerning tamoxifen's lingering effects on breast and endometrial cancer.