Early inspections revealed the damage at the renowned cancer specialist hospital was not as bad as first thought, bosses said.
Some hospital staff have moved back into part of the west London site, using an area as an administrative centre.
Cally Palmer, chief executive of the Royal Marsden NHS Foundation Trust, said: "We are working as quickly as we can to get the Chelsea hospital up and running and our main priority is to ensure continuity of care for our patients."
About 30 firemen remain at the site damping down hotspots.
The blaze, which at its height saw 125 firemen tackling the flames, had forced the evacuation of all staff and patients.
Prime Minister Gordon Brown visited some of the patients who were taken to the nearby Royal Brompton Hospital.
Nearly 80 inpatients as well as many outpatients had to scurry out of the hospital, covered in blankets against the near-freezing weather, to neighbouring hospitals and churches after the fire broke out at about 1:15 p.m. in a fourth floor room.
More than 800 staff, some still dressed in surgical scrubs, were evacuated as were two patients who were undergoing operations at the time.
One patient is believed to have been taken to another hospital suffering smoke inhalation.
It had initially been feared that key operating theatres would be affected and that two wards had been badly damaged.
A London Fire Brigade spokesman said it was too early to identify the cause of the fire.
The hospital was founded in 1851 and was the first in the world dedicated to cancer treatment and research.
