The Jewish General Hospital is refusing ambulances after a problem in its server room shut down all major computer systems.
Spokeswoman Megan Martin says the hospital expects its systems will be back up and running by 5 p.m.
The computer problem happened at about 7:30 a.m.
Martin said patient safety has not been compromised, and all scheduled surgeries will continue.
Some non-urgent services such as elective radiology are not available for the day.
Ambulances will be redirected to other health-care centres until at least 4 p.m.