Five years of Tamoxifen (Standard TAM) is a common treatment option for early-stage, hormone receptor positive (HR+) breast cancer (BC). Extending Standard TAM by 5 additional years (Extended TAM) can improve survival and BC recurrences. In postmenopausal women, the use of extended aromatase inhibitors (Extended AI) after Standard TAM is an alternative to Extended TAM. This study examines the cost-effectiveness (CE) of extending Standard TAM with Extended TAM vs. Extended AI in postmenopausal HR+ early-stage BC patients. Three treatments were assessed: (1) Standard TAM; (2) Extended TAM; (3) Extended AI through a Markov model using a Canadian health system perspective, lifetime time-horizon, quality adjusted life years (QALYs), and a 5 % discount rate for future costs and utilities. Incremental cost-effectiveness ratios (ICERs) were calculated, and the impact of parameter uncertainty was assessed through probabilistic sensitivity analyses (SA) using conventional CE thresholds. The estimated total per person costs in 2012 Canadian dollars [$1.00 CAD = $0.99 US 2012] were the least for Extended TAM ($8,623 CAD) and most for Extended AI ($9,432 CAD). Extended AI was the most effective regimen, while Standard TAM was the least. Extended AI was cost-effective at conventional thresholds vs. Extended TAM (ICER: $3,402 CAD/QALY) which was robust to the SA. This study suggests that Extended AI and Extended TAM result in improved QALYs and lower healthcare costs vs Standard TAM. Extended AI results in the greatest improvement in QALYs and is the most cost-effective treatment alternative despite its higher drug costs.