Eight women given the all-clear after being screened for breast cancer are now being treated for the disease after a review of the service in Cumbria.
In July, North Cumbria University Hospitals NHS Trust announced breast screening was being temporarily halted for a major review to be carried out.
It involved 1,600 women who had had further tests following routine mammograms since April 2007.
The trust has apologised for any distress caused.
Officials at the trust decided to have the review when data showed that among women who were called for a second screening, cancer was detected in fewer cases than would be expected.
About 1,600 films were sent for review to external screening centres.
So far, 1,477 have been returned with 1,302 needing no further tests.
Another 130 were told they would benefit from further assessment. Of those, 49 have been seen so far, with 40 not needing any further treatment.
Of the eight women who were told they needed treatment, three have already started, another three have planned treatment and two are having a plan developed.
A further patient has non-invasive cancer.
Trust chief executive Carole Heatly said: "Our trust fully understands and appreciates the distress and anxiety this issue will have caused and apologies to all concerned.
"We are undertaking every measure possible to ensure that we provide the best high-quality service."
Screening is still suspended but the trust said it hoped it would be restored as soon as possible following the review.
Meg McArthur, from the charity Breakthrough Breast Cancer, said: "Any type of misdiagnosis is extremely worrying and we hope that appropriate measures are put in place as soon as possible to reassure women and to ensure that this does not happen again.
A helpline has been open since the review began. It is available Monday to Friday from 0900-1700 on 01228 608289.