OBJECTIVES: We compared the effectiveness of bisphosphonates combined with activated vitamin D administered for therapy of aromatase inhibitor-induced osteoporosis after a breast cancer operation and primary postmenopausal osteoporosis through propensity score matching. METHODS: Forty-eight postmenopausal patients with estrogen receptor-positive early breast cancer, who had postoperative adjuvant treatment with aromatase inhibitors and whose T-score of bone mineral density (BMD) decreased below -2.5 (AI group), and 48 patients of primary postmenopausal osteoporosis (PO group) enrolled in this retrospective observational study. They were administered monthly risedronate or minodronate, and daily alfacalcitol or eldecalcitol were combined. Their BMD (L2-4, L-BMD), serum-corrected calcium, serum phosphate, tartrate-resistant acid phosphatase 5b (TRACP-5b), bone alkaline phosphatase (BAP), estimated glomerular filtration rate, urine calcium/creatinine ratio, intact-parathyroid hormone, and 25-hydroxy vitamin D were measured before treatment and until 24 months. RESULTS: L-BMD values increased with time compared with the baseline values in each group, and there was no significant difference in the groups. Percentage value of TRACP-5b decreased rapidly after 6 months and maintained low level until 24 months in both groups. Percentage value of BAP in the AI group decreased continuously until 24 months. In contrast, the percentage change in the PO group plateaued after 6 months. CONCLUSIONS: It is suggested that monthly oral bisphosphonate combined with activated Vitamin D is an effective therapy to increase BMD in the aromatase inhibitor-induced osteoporosis after breast cancer operation. Monitoring of kidney function and concentration of Ca in blood and urine may be necessary.