The group says it filed a freedom of information request with the Ministry of Health and other branches of government on Monday for records regarding so-called medical tourism going back five years.
It says two Toronto hospital groups already let non-Canadians pay for medical treatment.
The nurses group say more needs to be known about what it calls an "attack on Medicare" that sees money rather than patient need determine who gets help.
And they're warning that medical tourism will redirect resources away from Ontario patients to those from abroad.
The association represents almost 40,000 registered nurses, nurse practitioners and nursing students in the province.