The health board, which is mired in controversy over waiting lists, farmed out work to Murrayfield Hospital to hit targets.
But Spire Healthcare, which owns the private hospital, used many of the same consultants as NHS Lothian to carry out the work.
The Scottish Liberal Democrats are now calling for Health Secretary Nicola Sturgeon to make a statement before parliament on the matter.
An internal audit by NHS Lothian found a number of concerns about Spire.
It said there were potential “conflicts of interest” where “consultants and other staff employed by NHS Lothian can benefit from treating NHS Lothian patients privately.”
NHS Lothian paid Spire Healthcare £3.2 million a year, in a three-year contract starting in 2008.