Medicare officials say about 20 percent of senior citizens in Colorado -- or about 112,000 people haven't yet signed up for the new Medicare prescription drug program.
After midnight tonight, people who enroll face a one percent increase in premiums for every month they wait to sign up.
Officials with the federal Centers for Medicare and Medicaid Services say the majority of people who haven't enrolled yet are likely low-income elderly who qualify for a federal subsidy that covers the monthly premiums.
There are walk-in clinics planned today to help people enroll in Medicare Part D, which gives seniors the option to join one of many private insurance plans.
In Colorado, 18 private companies are offering 43 approved plans with monthly premiums ranging from about nine dollars to nearly 66 dollars.
The Bush administration has rejected calls to extend the sign-up deadline, saying a time limit was necessary to encourage as many people as possible to enroll.
Federal statistics indicate that about 417,600 of the 529,300 eligible Coloradans have signed up.