Inquiry officials say whistleblowing witness Ken Pereira fell down the stairs outside his home — which they say was a simple accident.
They say he suffered head trauma and has been ordered by doctors to rest until next Monday. Inquiry hearings will resume the following day, after the Thanksgiving holiday.
Pereira has been a talkative witness, so far.
He has been testifying about corruption inside FTQ-Construction, a wing of Quebec's most powerful labour union.
He has described how Hells Angels sympathizers and men connected to the Italian Mafia infiltrated the union and wrestled for control of a popular invesment fund.
Tuesday's incident occurred just moments before an inquiry investigator was to pick Pereira up to transport him to the downtown office tower where his testimony was to continue.