Background: Medical studies have shown that switching to exemestane after 2-3 years of adjuvant treatment with tamoxifen is effective when looking at overall survival. No cost effectiveness study of exemestane has been conducted in the German health care context. Patients and Methods: To assess the cost effectiveness of switching to exemestane vs. continued tamoxifen therapy for early-stage breast cancer, a Markov model was developed. The model population was set as postmenopausal women who are in remission from early-stage breast cancer. Upon model entry, either a continuing daily therapy with 20 mg tamoxifen or a switch to 25 mg exemestane for the next 2-3 years takes place. The model takes a German health care perspective. Results: The total incremental costs of exemestane on a lifetime basis are 4,195 Euro, resulting in an incremental cost effectiveness ratio of 17,632 Euro per additional quality-adjusted life year (QALY), or 16,857 Euro per life year gained. Incremental costs per disease-free year of survival are 12,851 Euro. Probabilistic sensitivity analyses proved the robustness of these findings. Conclusion: Compared to extended tamoxifen therapy, switching to exemestane after 2-3 years turned out to be a cost-effective strategy in adju-vant therapy for early-stage breast cancer in postmeno-pausal women within the German health care context. © 2009 S. Karger GmbH, Freiburg.