The Belfast Trust has said it is "truly sorry" for having to recall 2,500 patients after a probe into the work of a consultant neurologist.
The recall comes after an independent review of patient notes relating to the work of Dr Michael Watt.
It follows a review of patient notes by both the Trust and the Royal College of Physicians.
The consultant is currently not treating patients, but is still an employee, the Trust said.
Concerns were raised by other doctors about Dr Watt's management of patients, particularly his treatment plans and diagnoses.
The patients recalled were being treated for a wide range of neurological conditions, including epilepsy and MS, and are aged 14 and up.
Dr Mark Mitchelson, medical chair of division, said: "I fully understand this will cause significant anxiety to many patients and their families and for that we are truly sorry.
"Other doctors raised concerns with the Trust regarding the care and treatment provided by Dr Michael Watt to a small number of patients.
"As a result of our own review and a further review carried out by the Royal College of Physicians, we would like to invite a number of his patients back for a review appointment to assure them, their families, and ourselves that they are receiving the best treatment.
"The recall of such a large number of patients is so that we can be confident and thorough in ensuring that patients are having the best possible care.
"To ensure patients are seen as quickly as possible we have set up a large number of additional clinics and we intend to see all patients within the next 12 weeks.
"Again, we are truly sorry for the anxiety this will cause."
The Belfast Trust said it has contacted every patient it would like to review.
The Trust is asking patients to make contact so that an appointment can be made.
For anyone who may be worried and is seeking further support, an advice line has been set up.
The telephone number is 0800 980 1100 and lines will be open weekdays from 9am to 9pm and weekends from 9am to 5pm.