Brain samples contained evidence of a viral infection
Scientists analysed a type of tumour called a medulloblastoma - the most common type of malignancy found in the brains of children.
The virus infects at least 70% of children early in life, but in most cases lies dormant.
JCV is already known to cause a deadly disease of the nerves called progressive multifocal leukoencephalopathy (PML) - particularly common in Aids patients.
Uncontrolled growth
The researchers, from Temple University in Philadelphia, believe that JCV may stimulate the rapid and uncontrolled cell growth typical of cancer.
Lead researcher Professor Dr Kamel Khalili said: "We have a virus in our body which may have the potential to cause tumours."
However, he stressed the research was not suggesting every single brain tumour is caused by the JC Virus.
"What we are saying is that the virus can be detected in a good number of the human brain tumours we sampled."
Two types
Two types of protein produced by JCV were found in the tumour samples. They are known as agnoprotein and T antigen.
Evidence of agnoprotein was found in 69% of 16 medulloblastoma samples, while evidence of T antigen was found in 65% of 20 medulloblastoma samples.
T antigen may cause brain tumours in part by blocking the action of other proteins that play a role in suppressing the development of tumours.
The role of agnoprotein in the development of brain tumours is unknown.
Dr Howard Fine, of the US National Cancer Institute and the National Institute of Neurologic Disorders and Stroke said the presence of the protein in tumour cells may simply be a sign of an old infection not directly linked to the development of cancer.
The research is published in the Journal of the US National Cancer Institute.