Some NHS Wales websites are inactive after a failure, the health service said.
Some unavailable services include My Health Online, which allows patients to book appointments and order repeated prescriptions.
The Welsh Blood Service site has been previously affected, but has since been restored. Access to key information on NHS Wales Direct is not affected.
“We are working hard to restore services on all websites as soon as possible,” said a spokesman.
Hywel Dda, Powys and Aneurin Bevan and the Velindre Trust all confirmed that their services were inactive.
The NHS Wales IT Service apologized and advised patients to contact their GP practice directly.