Nurses and midwives have voted to go on strike over staff shortages and pay.
Members of the INMO voted in favour of industrial action by 95%.
The executive will meet on January 7th and 8th to discuss the result and decide their next steps.
40,000 members of the INMO voted today on whether or not to strike, with the Psychiatric Nurses Association holding a similar ballot of its 5,000 members.
The vote was called in a row over pay and staffing in the health service.
The INMO has accused the Government of exaggerating the cost of granting its 12% across-the-board pay claim.
Hospitals could face 24 hour stoppages, with just emergency cover provided.
The stoppages could escalate in the event of a prolonged dispute.