[en] This study aims to estimate the potential clinical and economic implications of therapeutic adherence to bisphosphonate therapy. A validated Markov microsimulation model was used to estimate the impact of varying adherence to bisphosphonate therapy on outcomes (the number of fractures and the quality-adjusted life-years [QALYs]), health-care costs, and the cost-effectiveness of therapy compared with no treatment. Adherence was divided into persistence and compliance, and multiple scenarios were considered for both concepts. Analyses were performed for women aged 65 years with a bone mineral density T-score of -2.5. Health outcomes and the cost-effectiveness of therapy improved significantly with increasing compliance and/or persistence. In the case of real-world persistence and with a medical possession ratio (MPR; i.e., the number of doses taken divided by the number of doses prescribed) of 100%, the QALY gain and the number of fractures prevented represented only 48 and 42% of the values estimated assuming full persistence, respectively. These proportions fell to 27 and 23% with an MPR value of 80%. The costs per QALY gained, for branded bisphosphonates (and generic alendronate), were estimated at <euro>19,069 (<euro>4,871), <euro>32,278 (<euro>11,985), and <euro>64,052 (<euro>30,181) for MPR values of 100, 80, and 60%, respectively, assuming real-world persistence. These values were <euro>16,997 (<euro>2,215), <euro>24,401 (<euro>6,179), and <euro>51,750 (<euro>20,569), respectively, assuming full persistence. In conclusion, poor compliance and failure to persist with osteoporosis medications results not only in deteriorating health outcomes, but also in a decreased cost-effectiveness of drug therapy. Adherence therefore remains an important challenge for health-care professionals treating osteoporosis.