At its height, more than 100 firefighters battled the fire that ripped through the Royal Marsden Hospital in Fulham Road, central London.
All patients and staff were evacuated from the building and there were no injuries.
Many patients were taken to St Paul's Church in Onslow Square where they were being cared for by nursing staff.
Other patients were transferred to the Royal Brompton Hospital nearby.
Doctors and nurses worked in the cold, wintry weather to transport patients and medical equipment from the burning building to the Royal Brompton.
About half of the roof was destroyed, according to the London Fire Brigade.
The blaze started at 1320 GMT on Wednesday and carried on into the night, with fire crews damping down by morning.
Professor Ray Powles, former head of haematooncology at the hospital, said the damage to the hospital would be a "huge step back" for cancer treatment.