The INMO has been comparing trolley figures each January since 2006.
Over 670 patients have been left waiting on hospital trolleys in the midlands, so far this month.
A review of details, compiled by the Irish Nurses' and Midwives' Organisation, compares trolley figures from every January between 2006 and 2020.
2018 was the worst January in the last 14 years for overcrowding, according to the INMO's Trolley Watch Analysis,
Just under 1,500 patients were left waiting on hospital trolleys in the midlands - compared to over 12,000 nationally.
It was the worst year on the records for the Midlands Regional Hospitals in both Mullingar and Tullamore - with 635 and 556 waiting respectively.
Meanwhile, Portlaoise Regional Hospital peaked in 2017, with 477.
2008 saw the lowest numbers for the midlands, at 100 - as no figures are available for Tullamore Hospital in January 2007.