The Royal Marsden Hospital has reopened to out-patients after fire swept through the roof, damaging two wards and five operating theatres.
The fire started on the fourth floor and destroyed much of the roof
About 350 staff and patients were forced to leave the world-renowned treatment centre in west London when the fire broke out on Wednesday.
The hospital's chemotherapy and radiotherapy departments began operating as normal on Monday morning.
Investigations are continuing into the cause of the fire.
Chief executive of the Royal Marsden NHS Foundation Trust, Cally Palmer, said she was delighted to be able to open up part of the hospital so soon after the fire.
"Our first concern is always for our patients and our main aim is to reassure them that we are doing everything that we can to get back to as normal a service as possible," she said.
Is is hoped that 21 in-patients who were moved to the nearby Royal Brompton Hospital will return to the Royal Marsden over the next two days.
The hospital planned to open three wards on Monday, in areas of the hospital that were not damaged by the fire, with further wards opening in the next few weeks.
Ms Palmer said they were "deeply grateful to the emergency services, the Royal Brompton and all those who have supported us at this difficult time".
"The fact that we can open our doors less than a week after this serious fire is a tribute to the immense hard work of the whole team," she said.