Aromatase inhibitors are rapidly becoming the cornerstone of endocrine treatment for advanced disease and are now also used as adjuvant treatment in early-stage disease. The objective of this study was to assess the cost-effectiveness of adjuvant treatment with exemestane versus tamoxifen for early-stage breast cancer after 2-3 years treatment with tamoxifen in Sweden. The results are based on findings in the Intergroup Exemestane Study (IES). IES was a randomized controlled trial in which postmenopausal women who had received 2-3 years of tamoxifen therapy following primary treatment of early-stage breast cancer, were randomized to either continue on tamoxifen therapy or be switched to exemestane therapy. The results showed a disease-free survival hazard ratio of exemestane relative to tamoxifen in IES of 0.69. A Markov state-transition model was developed to simulate consequences after the end of the clinical trial, and to integrate the trial data with external data on mortality, costs and quality of life specific for Swedish women. The cost per QALY gained was about euro 20,000 in the base case analysis without inclusion of consequences of coronary heart disease. Inclusion of these events increased the cost-effectiveness ratio to about euro 31,000. This means that, based on our assumption, sequential exemestane treatment in early breast cancer is a cost-effective option compared with tamoxifen alone, although more long-term data on overall survival and consequences of adverse events would be valuable to increase the validity of the analysis further.