Around 18,000 people will get a letter from hospitals in Galway and Roscommon in the coming days to see if they still want an outpatient appointment.
It is part of a plan by the Galway and Roscommon University Hospital Group to cut waiting times and reduce the number of "no shows" at outpatient clinics.
Last year, around 23,000 patients failed to turn up to see doctors in the four hospitals that make up the group.
People added to the waiting list in 2012 will be contacted in the coming days.
They will be asked to telephone a dedicated line to update contact details and confirm whether they still need an appointment.
The Group's Chief Operating Officer, Tony Canavan, said this is the latest part of a validation project which began last November.
Waiting times have been identified as a major challenge at the hospitals involved.
Mr Canavan said efforts are under way to ensure no patient will wait longer than 12 months to be seen.