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.
More top news
Organisers say the move is due to "unprecedented demand".
The wintry snap we enjoyed last week now seems like a dim and distant memory,as we return to dull, damp and mild conditions!
Police boss says cyber criminals from Russia and Eastern Europe are attacking businesses across North Wales