QuoteEight cancer operations were cancelled after copper cabling was stolen from a hospital in the Vale of Glamorgan.
Eighty one patients were affected on Wednesday by the theft from a back-up generator at Llandough hospital near Cardiff.
Hospital managers said Tuesday's theft of 100m of cabling was "dangerous and irresponsible".
A new back-up generator is being tested before routine surgery can resume.
It was too late for Wednesday's morning list of 36 operations, including two breast cancer operations, which was cancelled as a precaution.
Another 45 operations scheduled for the afternoon were also cancelled after a meeting.
The University Health Board chief executive Jan Williams called the theft "mindless".
She said: "NHS staff work tirelessly to care for some of the most vulnerable members of our communities and it is depressing to note that, for these thieves, the monetary value of copper is of more consequence."
Llandough deals with non-emergency operations, including orthopaedic and breast surgery.
Ms Williams said the cancellations were "traumatic" for the two breast cancer patients in particular but she said their operations would be rescheduled as quickly as possible.
Health managers are taking steps to provide secondary back-up facilities but these were not in place to allow afternoon operations to take place.
Replacing the cables could cost up to £20,000.
The theft was reported to South Wales Police just after 14.00 GMT on Tuesday.
I wonder have their been any 'Travellers' or other immigrants, famous for relieving properties of their metal goods, in the area?