A cancer charity says it is "deeply saddening" that patients are attending hospital appointments or treatment on their own.
In a survey of 2,000 patients, 46% said they had attended at least one chemotherapy appointment alone and 60% had been for radiotherapy alone.
Many patients said they did not want to be a burden on friends and family.
Having a friend or relative for support made a huge difference, Macmillan Cancer Support said.
Jacqui Graves, head of health and social care at Macmillan Cancer Support, said going for an operation or through treatment alone could have a devastating impact on patients.
"There is no-one to offer a warm hug if they feel distressed, no-one to remind them of practical needs like taking anti-sickness medication if they have chemotherapy.
"There is no-one to act as their advocate, asking the doctor the questions they might not have thought to ask because they are too overwhelmed, or to calm them down if they are stressed or anxious."
In the survey, one in 20 people with cancer said they had been to a hospital appointment alone because they had no-one to ask to go with them.
More than half of the people surveyed who had had surgery for their cancer had also been to at least one of their appointments alone.
Jacquie Graves added: "It is deeply saddening that cancer patients are facing one of the most frightening moments of their lives - going for an operation - without having anyone with them to offer support."
However, 60% said they did not consider it necessary for anyone to go with them, and some people said they wanted to go alone.
Macmillan said the figures suggested that about 270,000 people could be attending cancer-related hospital appointments on their own in the UK.
The charity has set up a website called The Source, which contains tips on how best to support family and friends with cancer so they do not face their treatment alone.