Quality problem or issue: In 2005 we were informed about the possibility of a systemic error in pathology diagnoses of 7400 histological tests done between 4 and 6 years earlier. Initial assessment: We determined to undertake a lookback and apply principles of open disclosure to inform the affected community of 200 000 people. Choice of solution: The lookback included subjecting all cases to independent pathology review. The public announcement of the review included an unreserved apology and took place before the results of the re-examination of the pathology specimens were known. Implementation: The lookback involved the simultaneous implementation of five critical elements: leadership and governance, risk assessment and planning, implementation of the independent review, procedures for patient care and communication and open disclosure. Protocols were developed to care for those patients whose original test results were found to be incorrect. Evaluation: The original result for >200 patients was incorrect, and 38 had experienced clinical consequences. There was no public panic as a result of the wide open disclosure. Few related legal claims or complaints were made. The impact of the pathology diagnostic error has continued to 2011 for some patients. Lessons learned: Openly disclosing a risk of widespread error meant the community could be supported with information and medical management as needed. Credentialing and peer-review processes for senior staff must be precise and collegiate. Sometimes action has to take place even when the risk is ill defined. There are five critical elements in planning and implementing a large-scale lookback.