No, you do not lose your accumulated Delta SkyMiles if you cancel your co-branded American Express card, as the miles are stored in your Delta SkyMiles loyalty account, not on the credit card itself. Once miles are earned through spending and deposited into your SkyMiles account, they are yours to keep and, under Delta's current policy, they never expire. However, you will lose the specific "cardmember perks" associated with that account, such as the "15% off" (TakeOff 15) discount on award bookings, the free first checked bag benefit, and priority boarding. Additionally, if you have any unused Companion Certificates tied to a Platinum or Reserve card, those are typically forfeited upon closing the account as they require an active card to be redeemed. To preserve your credit score and relationship with the bank, consider "downgrading" to the no-annual-fee Delta Blue card instead of canceling entirely.