Medicare Part D has had important implications for patient outcomes and treatment costs among beneficiaries with congestive heart failure (CHF). This study finds that improved medication adherence associated with expansion of drug coverage under Part D led to nearly $2.6 billion in reductions in medical expenditures annually among beneficiaries diagnosed with CHF and without prior comprehensive drug coverage, of which over $2.3 billion was savings to Medicare. Further improvements in adherence could potentially save Medicare another $1.9 billion annually, generating upwards of $22.4 billion in federal savings over 10 years.