Cardiff and Vale University Health Board said that "torrential rain" yesterday caused water to enter the Children's Hospital for Wales.
The hospital has been "temporarily evacuated" while the extent of water damage is assessed.
It says 18 patients have been safely moved to child health areas in the main University Hospital of Wales building, and are being cared for by staff from the children's hospital.
Our teams were superb in swiftly moving children out of the affected areas and identifying safe and appropriate areas within the main hospital as temporary accommodation.
No-one was injured in the incident and everyone has remained calm, with staff helping patients and their families settle in to their new environments. Our estates team is currently assessing the damage but it is likely the Children’s Hospital will be out of action for the remainder of this week.
Paediatric emergency and elective inpatient services have not been affected.
The cause of the flooding is currently being investigated.
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There is likely to be a good deal of cloud around today, thickest in the southeast, where outbreaks of rain are possible at times.
A dry night with clear spells for most, especially towards the north where it will become rather chilly with some mist patches possible.