Northern Ireland is carrying out a huge hospital waiting list audit.
Tens of thousands of people are being sent letters as part of a validation exercise to update lists.
The Royal College of GPs (RCGP) has advised patients to respond to their letters within two weeks to avoid the risk of losing their place on the list.
Almost 13,000 letters have already been sent as part of the audit, which is in its earliest stages, BBC News reports.
Among those already contacted are patients waiting 190 weeks for dermatology and three years for routine ENT appointments.
Nearly 300,000 people in Northern Ireland are on a waiting list for a first appointment with a consultant, according to recent Department of Health figures.
A total of 299,436 patients were waiting at the end of June, which is 23,500 more than the previous year.
More than a third of patients have waited for more than a year.
Dr Grainne Doran, RCGP NI chairwoman, warned that there is a significant risk patients will lose their appointment if they do not reply.
She said patients may no longer need appointments because they have sought private treatment or their symptoms had cleared up since being placed on the list.
An HSCB spokesperson said trusts were required to routinely undertake waiting list audits to ensure patients still require appointments and that contact details for patients are up to date.