Introduction: Dutch policy regulations require outcomes research for the assessment of appropriate drug use and cost-effectiveness after 4 years of temporary reimbursement. We investigated whether outcomes research reduced policymaker uncertainty regarding the question whether the costs are worth public funding. Methods: Our cohort study included 139 patients with relapsed/refractory multiple myeloma who were treated outside of a clinical study; 72 received bortezomib and 67 did not receive bortezomib. Detailed data were retrospectively collected from medical records in 38% of Dutch hospitals. Results: All patients received second-line treatment; 65%, 40%, and 14%, received three, four, or five or more lines of therapy. Neither a specific treatment sequence nor an appropriate comparator could be identified because of large variation in regimes. Kaplan-Meier curves showed an increased overall survival (mean [median] 29.5 [33.2] vs. 28.0 [21.6] months) for patients treated with bortezomib (Wilcoxon P ¼ 0.01). Total mean costs were €81,626 (range €17,793–€229,783) and €52,760 (range €748–€179,571) for patients receiving bortezomib and patients not receiving bortezomib, respectively. Patients treated with bortezomib, however, were not comparable to other patients despite attempts to correct for confounding. Therefore, it was impossible to develop a feasible model to obtain a valid incremental cost-effectiveness estimate. Conclusions: It was possible to develop evidence on bortezomib’s use, effects, and costs in everyday practice. Much uncertainty, however, remained regarding its osteffectiveness. Policymakers should carefully consider whether outcomes research sufficiently decreases uncertainty or whether other options (e.g., finance- and/or outcomes-based risk-sharing arrangements) are more appropriate to ensure sufficient value for money of expensive drugs.