Thirteen patients who were wrongly given the all-clear for bowel cancer were found to have the disease.
They include a man who has now died of the illness, an investigation report has revealed.
All of the patients had undergone a colonoscopy - an invasive test of the bowel - at Wexford General Hospital.
They were participating in BowelScreen, the national bowel screening programme for people aged 60 to 69.
A report of a review into the incident, released last night, said the cancers were probably missed during the original test.
The missed cancers have emerged following a recall of 615 patients at Wexford General Hospital in 2013 and 2014.
Six of the patients were from Wexford, and seven were from carlow-Kilkenny.
One of the patients has died of bowel cancer and others have been treated for the disease.
The recall of hundreds of patients followed a discovery by BowelScreen that two patients who had a colonoscopy procedure in Wexford General Hospital in 2013 were discovered to have cancer in October and November 2014.
The revelation is a setback for the programme which currently has an uptake of just 45pc among the 60 to 69 age group .
Hospitals which are given the work by BowelScreen must be accredited and consultants who carry out the work have to agree to have their performance reviewed on a continuous basis.
A spokeswoman for the screening programme said last night that a number of recommendations to improve quality assurance are now in place.
The patients were diagnosed with caecum cancer, a type of colon cancer, but the signs and symptoms of this disease are sometimes elusive.
The allegation is that the method of colonoscopy used by the consultant was faulty and was not invasive enough.
This is disputed by the consultant who said they achieved an intubation rate of 91pc. The doctor no longer carries out colonoscopies at the hospital.
A spokeswoman said last night: "Screening is the most effective way to detect bowel cancer, including pre-cancerous changes.
"In its first round completed in 2015, BowelScreen screened over 196,000 people, performed over 8,000 colonoscopies and detected 517 cancers, making it a truly lifesaving programme. In addition, approximately 12,000 abnormal growths (polyps) were removed, which could have developed into cancer over time. Since this incident, we have reviewed our quality assurance indicators."