Medicare Advantage (“MA”) beneficiaries currently face higher out-of-pocket costs for behavioral health services than Traditional Medicare beneficiaries do. That may change soon under a rule recently proposed by the Centers for Medicare & Medicaid Services (“CMS”). This article summarizes how the Proposed Rule would change cost-sharing for behavioral health services and the potential implications for plans and behavioral healthcare providers. We suggest that comment is needed on a critical assumption of the Proposed Rule—that permitting beneficiaries to pay less for covered services will lead them to use those services more often, without causing significant losses to MA plans.