Vital blood transfusions may be at risk because of low stocks according to the Welsh Blood Service.
The service is appealing for more people to donate blood over the festive period
Donations traditionally fall during the winter months because of infections like colds and viruses but the service warns more blood is desperately needed.
It is calling for help with young cancer patients who are due to have their treatment before Christmas.
"Patients with cancer and leukaemia need regular transfusions," said the service's spokeswoman Tracey Sampson.
"Without these vital transfusions they would not be able to continue their treatment, leave hospital and go home for Christmas.
"Most of us love to spend Christmas with our family and friends - for some people this will only be possible with the help of blood donors," Ms Sampson added.
The service is highlighting the case of four year old Carys Orrell, who was diagnosed with leukaemia in January.
She has been undergoing chemotherapy in the children's ward at the University Hospital of Wales in Cardiff.
In the last nine months Carys has needed many transfusions and her father Tony Orrell said: "Without donations, Carys and many children like her would not be able to undergo treatment and I dread to think what would happen."