Error leads to missed hospital appointments in Scunthorpe, Grimsby and Goole
Information that 100 patients were left waiting for hospital appointments only came to light after a council used legal powers to make it public.
Problems with a new referral system means some GPs' requests for hospital consultations in Scunthorpe, Grimsby and Goole have not been recorded.
The NHS trust is asking patients who have been referred by their doctors between January and October 2016 to contact them.
An investigation has been launched.
The interim Chief Executive of the Northern Lincolnshire and Goole NHS Foundation Trust (NLAG) has admitted it kept news of missing referral data quiet as they tried to find an administrative solution.
Councillor Holly Mumby-Croft, of North Lincolnshire Council's Scrutiny Committee, said: "I took the decision to invoke legislation to require the chief executive of NLAG and the chief officer of the clinical commissioning group to attend scrutiny because I was sufficiently concerned that this needed to be discussed."
Northern Lincolnshire and Goole NHS Foundation Trust (NLaG) said it had started a new centralised referral unit in October.
After complaints it reviewed the system and found that 111 patients were missing their appointments, leading the trust to believe more people could have been omitted.
The trust's interim chief executive, Richard Sunley, said: "We are currently carrying out reviews and checks of all of our waiting lists.
"As part of this work we believe there may be patients who were referred before the new central referral team was put in place, who have not been added to our waiting lists."