Nancy Morgan, a "writing clinician", approached patients waiting in a clinic at a cancer centre in Washington DC.
Half those who took part said the exercise changed the way they thought about the illness, according to the journal The Oncologist.
Younger people, and those recently diagnosed, were most likely to benefit.
Thoughts and feelings, or the cognitive processing and emotions related to cancer, are key writing elements associated with health benefits
Nancy Morgan, Lombardi Center
Ms Morgan developed her role as part of the Arts and Humanities Program at the Lombardi Center.
Her "expressive writing" exercise, lasting just 20 minutes, posed questions to leukaemia or lymphoma patients about how the cancer had changed them and how they felt about those changes.
When those taking part were contacted again a few weeks later, 49% said that the writing had changed their thoughts about their illness, while 38% said their feelings towards their situation had changed.
While there was no evidence of direct impact of the session on their illness, where the patients had reported greater changes in their mindset during the writing, this could be linked to more positive reports of quality of life given to their doctors during follow-up appointments.
Ms Morgan said: "Thoughts and feelings, or the cognitive processing and emotions related to cancer, are key writing elements associated with health benefits, according to previous studies.
"Writing only about the facts has shown no benefit."
Dr Bruce Cheson, the head of haematology at Lombardi, said: "I'm pleased to see that so many of our patients were interested in this kind of therapy.
"Our study supports the benefit of an expressive writing program and the ability to integrate such a program into a busy clinic."