Health chiefs say two hospitals in Cardiff and the Vale of Glamorgan are under significant pressure.
It follows a big spike in the number of people turning up at the emergency unit at the University Hospital of Wales in Cardiff - with Llandough Hospital also affected.
Over 200 more people than expected have turned up at the emergency unit in Heath in the last 72 hours.
The Cardiff and Vale health board has told Bro Radio News that Llandough is still providing urgent care for those who need it and supporting UHW in an effort to ease some of the pressure.
A spokesperson for CAVUHB said: "This influx in attendances has caused significant pressure across both University Hospital of Wales and University Hospital Llandough and has resulted in incredibly long waits in the department and for admission to a hospital bed."
The health board is urging the public to only attend the emergency unit in Heath if it's necessary and to be prepared for longer waits than usual.
When asked by Bro Radio News about the impact on Llandough, a CAVUHB spokesperson said: "University Hospital Llandough is continuing to provide care for patients who require urgent care and have been referred by their GP practice and is supporting the University Hospital of Wales in an effort to ease some of the pressure."
They added: "We are urging the public to choose services appropriately and only attend the Emergency Unit if it is an emergency. If you do need to attend the Emergency Unit, please be aware that you may wait longer than usual."
"If you are unsure what healthcare support you need, NHS 111 Wales is available online and over the phone for healthcare advice, guidance, and for booking appointments at the out-of-hours service or the Minor Injuries Unit. Your Primary Care team in the community may also be able to support."
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