[en] ABSTRACT Objective: To estimate the impact of medication adherence on the cost-effectiveness of mass-screening by bone densitometry followed by alendronate therapy for women diagnosed with osteoporosis. Methods: A validated Markov microsimulation model with a Belgian health-care payer perspective and a lifetime horizon was used to assess the cost per quality-adjusted life year (QALY) gained of the screening/treatment strategy compared with no intervention. Real-world adherence to alendronate therapy and full adherence over 5 years were both investigated. The real-world adherence scenario employed adherence data from published observational studies, and medication adherence was divided into persistence, compliance, and primary adherence. Uncertainty was investigated using one-way and probabilistic sensitivity analyses. Results: At 65 years of age, the costs per QALY gained because of the screening/treatment strategy versus no intervention are euro32,008 and euro16,918 in the real-world adherence and full adherence scenarios, respectively. The equivalent values are euro80,836 and euro40,462 at the age of 55 years, and they decrease to euro10,600 and euro1229 at the age of 75 years. Sensitivity analyses show that the presence of the upfront cost of case finding has a substantial role in the impact of medication adherence on cost-effectiveness. Conclusion: This study indicates that nonadherence with osteoporosis medications substantially increases the incremental cost-effectiveness ratio of osteoporosis screening strategies. All aspects of medication adherence (i.e., compliance, persistence, and primary adherence) should therefore be reported and included in pharmacoeconomic analyses, and especially in the presence of the upfront cost of case finding (such as screening cost).