The West Island Health and Social Services Centre says a review of exams conducted on 684 patients between 2009 and 2012 are incomplete.
The examinations were carried out at the Lakeshore General Hospital.
Dr. Richard Germain, of the regional health and social services centre, says the examinations were all carried out by the same doctor.
That doctor no longer works at the Lakeshore General.
The hospital says patients have been advised by letter that they should be examined again.
Hospital hours will be extended to accommodate the patients and agreements have also been reached with other Montreal-area hospitals to treat them as well.
The facility says the re-testing is being done as a preventive measure so the patients concerned don't have any doubts about the state of their health.
Colonoscopies are used to detect inflammation, lesions, bleeding or tumours in the colon and are a preventative measure against colon cancer.