Eastern Health says the records were inappropriately accessed by two clerks working in a rural clinic over the past two years.
They say one of the workers was fired as a result of the ongoing investigation while the other employee resigned.
Eastern Health says it received a complaint about the breach on July 16 and will contact patients involved, but it is not releasing the name of the clinic or the community affected by what happened.
The province's privacy commissioner has also been informed about the breach, the third one reported by the board this year.
Eastern Health said in June that an employee had been fired for inappropriately accessing the medical records of 122 patients.
Documents for a class-action lawsuit against Eastern Health were filed last month.
In March, three urine samples and eight vials of blood with patient names of them were discovered near a busy road in St. John's.