Staff working at three hospitals in West Yorkshire are starting a three-day strike in a row over plans to cut jobs and pay.
NHS clerical workers, including medical secretaries, receptionists and clerks, are walking out.
UNISON Regional Organiser Jim Bell said members had been forced to take the action because the management of Mid Yorkshire Hospitals NHS Trust is refusing to negotiate on their threat to jobs and pay.
The strike will affect the day-to-day running of the hospitals, affecting activities like locating relevant files and administering appointments.
There will be pickets at the affected hospitals to by the workers.