Purpose: To compare outcomes for hypothetical cohorts of postmenopausal patients with estrogen receptor-positive tumors that are ≤ 2 cm in size, with pathologically uninvolved axillary nodes, treated with radiation therapy plus tamoxifen versus tamoxifen alone after breast-conserving surgery. Methods: A Markov model was used to simulate patients' clinical course and estimate overall survival, recurrence-free survival, time with an intact breast, and death from breast cancer. Probabilities were derived from randomized trials and retrospective studies. Analyses were performed separately by age of diagnosis in 5-year increments from 50 to 80 years. Sensitivity analyses tested the stability of radiation benefit. Results: The modeled recurrence-free survival benefit of giving radiation therapy was 3.35 years for women who were 50 years of age at diagnosis, versus 0.61 years for women who were 80 years of age. In the 50-year-old cohort, radiation therapy resulted in additional 0.60 years survival, compared with 0.04 years among 80-year-olds. A 50-year-old woman who received radiation therapy plus tamoxifen was less likely to die from breast cancer than if she received tamoxifen alone (2.43% v 5.29%; relative-risk reduction, 54%). An 80-year-old woman had a 1.17% chance of dying from breast cancer if she received radiation therapy plus tamoxifen, versus 2.02% with tamoxifen alone (relative-risk reduction, 42%). Sensitivity analyses showed that the magnitude of benefit was strongly influenced by including unequal rates of developing distant disease after breast recurrence between the treatment arms and varying rates of local recurrence. Conclusion: The absolute and relative benefits of radiation therapy and individual patient preferences for different health states should be considered when selecting treatment.