More than 600 neurology patients in Northern Ireland were given an insecure diagnosis by a consultant, according to a report by Stormont's Department of Health.
Nearly 3,000 patients were recalled last year as part of an investigation into the work of neurologist Dr Michael Watt at Belfast's Royal Victoria Hospital.
It was the biggest ever medical recall in Northern Ireland.
Dr Watt, who worked at the hospital for more than 20 years, was suspended over concerns about possible misdiagnosis.
The patients were assessed to establish whether they received the appropriate diagnosis and treatment for a range of brain conditions including MS, Parkinson's disease, stroke and motor neurone disease.
One in five of them were told their condition had not been properly diagnosed.
A report, published yesterday, states that of the 2,953 patients recalled, reviewing clinicians found that 617 of them had been given a diagnosis that was "not secure".
A further 329 were told there was uncertainty over whether their diagnosis was secure.
A spokesman for the Department of Health stressed that a diagnosis considered to be "not secure" does not automatically equate to a misdiagnosis.
"It is highly regrettable that one person should have to go through this experience, let alone the large numbers that have been involved," he added.
The department also apologised for a delay in publishing the report.
It was initially scheduled for publication during the summer, but that was postponed due to what the department called "highly sensitive unforeseen circumstances."
Last month, Dr Watt offered his "sincere sympathy" to those patients affected.
