In high-income countries, tamoxifen has been replaced by aromatase inhibitors such as anastrozole in adjuvant hormone therapy for early breast cancer. These drugs target patients with positive hormone receptors, due to the better results achieved for disease-free survival. The cost-effectiveness of this treatment change has not been evaluated in middle income countries.The cost effectiveness of anastrozole vs tamoxifen was assessed during five-years of adjuvant treatment of hormone receptor-positive, post-menopausal early breast cancer patients.This is a literature-based analysis. The natural history of the breast cancer and the effects of treatment were modeled as a Markov process. Effectiveness was defined as disease-free survival. Transition probabilities for the disease and adverse effects were obtained from the literature. Costs were defined as the median of actual costs provided by health insurance companies and the Colombian National Cancer Institute expressed in 2007 Colombian pesos. Probabilistic sensitivity analysis was performed, along with one way sensitivity analysis was for the costs.Compared with tamoxifen, anastrazol results in an additional relapse-free period of 0.5 years. Each relapse-free year obtained by tamoxifen cost 27,210,604 pesos, or with anastrazol 37,071,337 pesos with a discount rate of 3%. The cost for a sustained-effects scenario were 23,617,400 pesos for tamoxifen and $16,140.282 for anastrazol.The use of anastrazol has an additional cost per relapse-free year of 7,521,363 pesos (2007). Therefore, for postmenopausal, early breast cancer hormone receptor positive women in Colombia, the cost-effective alternative is tamoxifen used as adjuvant therapy for five years.