One-in-five patients of a Belfast-based consultant neurologist were misdiagnosed, according to details of a report obtained by BBC Spotlight.
Dr Michael Watt is currently suspended from practising medicine.
It follows about 3,000 of his patients being given recall appointments last year.
The publication of a report into the outcome of the recall was supposed to have happened in the summer but it was cancelled at the 11th hour.
Dr Watt worked at the Royal Victoria Hospital as a neurologist diagnosing conditions like epilepsy, Parkinson's Disease and multiple sclerosis.
Last year, thousands of his patients were asked to come for a fresh assessment after concerns were raised about his care.
Many of them have been waiting anxiously for the publication of an outcomes report which would give them an overall picture of the recall.
It was due to be published by the Department of Health (DoH) in June but that was cancelled.
This week, some of Dr Watt's patients gathered at Stormont demanding the report be released.
Spotlight has obtained details of the report.
Figures within it indicate that out of almost 3,000 patients of Dr Watt, about 2,000 are said to have what is described as a secure diagnosis but more than 600 were found to have what is described as an insecure diagnosis.
In other words, one-in-five were told they had been misdiagnosed, which is potentially life-changing news.
A further 329 were told their diagnosis was now uncertain.
SDLP MLA Nichola Mallon has campaigned on behalf of the patients.
"Each one of those 600 people have just obviously had their lives completely turned upside down with devastating consequences, so it is truly shocking," she said.
The report also indicates a similar ratio had been put on the wrong prescription by Dr Watt - 545 patients.
A further 332 patients were told it's not clear if they were on the right medication.
The report also reveals that the recall of Dr Watt's patients has cost £1.5m.
The DoH has said it is frustrated by the delay in publishing its report but that is due to circumstances beyond its control.