They found as many as one in five young adults who have been cured of childhood cancer show symptoms of post traumatic stress disorder (PSTD).
Symptoms include periods of overwhelming anxiety.
They found 16 were clinically diagnosed as having PTSD at some time since their cancer treatment.
The rate was four times higher than that found among cancer survivors aged nine to 17.
Lead researcher Wendy Hobbie, a nurse practitioner at the Children's Hospital of Philadelphia, said: "Young adulthood is normally a time of increased vulnerability to stress, but especially so with cancer survivors, who are negotiating interpersonal relationships, their ability to have children, and need to focus on a career.
"Many of these young adults worry that their lives remain in danger."
Triggered by events
Ms Hobbie said anxiety could be triggered by events such as driving to a hospital, or smells associated with their treatment.
"These may be reminders potent enough to generate strong physical and emotional responses more than 10 years after treatment."
Ms Hobbie said PTSD may make cancer survivors hyper-sensitive about their health.
More worryingly, some cancer survivors may avoid seeking medical care because they cannot face being reminded of their previous treatment.
Recognition of this fact is particularly important as cancer treatment can produce a number of physical disorders later in life.
Ms Hobbie said: "These young adults need education, support and guidance to deal with the future."
Raising awareness
Dr Kate Law, head of research for the Cancer Research Campaign, said the study was "really important".
She hoped that the research would raise awareness of the problem among clinicians and psychologists.
Cancer patents are often offered counselling to cope with the immediate aftermarth of their treatment - but not years later.
Dr Law told BBC News Online: "With the best of intentions the medical profession lets these patients get on with their lives, but clearly some people remain vulnerable and it is important that we find some way to support them."
However, Dr Law said it was equally important not to swamp other patients with offers of continued care when they were doing their best to put their traumatic experience behind them.
The research is published in the Journal of Clinical Oncology.