Nearly 16,000 patients have been removed from waiting lists this year after a validation process.
More than 3,000 said they wanted to be removed from inpatient and day-case lists and nearly 13,000 asked to be taken off the outpatient list.
It followed an audit by the National Treatment Purchase Fund (NTPF).
Letters are normally issued to those who've been seeking an appointment for more than six months.
This year, the NTPF wrote to more than 195,000 patients.
The NTPF doesn't record the reasons why, but the Irish Patients Association says the audit suggests some patients may have got sicker and have presented to an emergency department instead.