Scottish health minister Sam Galbraith has demanded an explanation from health board managers on how a computer error led to 4,500 women failing to receive smear test recalls.
Highland health board detected the problem last month which had resulted in women born between 1931 and 1937 being dropped from the list of those invited for screening at the end of last year.
Managers identified deficiencies in the software in 1994, which they had not made public, but did not then realise the full extent of the problem.
Versions of the software, called Pinnacle, are in use in trusts across Scotland. The system was bought two years ago by Foremost Training. Bosses at Foremost said they had warned the Scottish Office then that the system needed updating.