Immortal Miles That Never Expire
Mileage Plan Miles are "immortal" and never expire* (subject to member's account remaining active for 3 years).

* If no mileage is credited to an account during the first nine months after opening, or if an account is inactive for longer than 3 years, the Alaska Airlines Mileage Plan reserves the right to delete an account and reassign the number. Once mileage has been deleted from an account, it will not be reinstated.