A new study adds to evidence that chronic fatigue syndrome doesn't have anything to do with xenotropic murine leukemia virus-related virus (XMRV), but the researchers point out that there's a caveat regarding American patients.

"Although our patient group was relatively small and we cannot formally rule out a role of XMRV, our data cast doubt on the claim that this virus is associated with chronic fatigue syndrome in the majority of patients," the researchers report in the Feb. 26 online edition of the BMJ.

No one knows what causes chronic fatigue syndrome, which is thought to affect millions of people around the world. But a recent U.S. study found signs of XMRV in two-thirds of patients with the condition.

However, other studies, including this most recent one, suggest that there's no connection between the virus and chronic fatigue syndrome.

In the new study, researchers from Radboud University Nijmegen Medical Center in the Netherlands examined DNA from 32 Dutch patients with chronic fatigue syndrome and 43 healthy people.

The researchers found no evidence of the virus in either group.

The reasons for the findings in the study of the American patients are unclear, but in a commentary that accompanies the new report, researchers from Imperial College and King's College London said it's possible that the virus may be one of several linked to an outbreak of chronic fatigue syndrome in the United States in the mid-1980s.

"The current debate will still bring critical attention to the causes of chronic fatigue syndrome, and XMRV may turn out to be important in the pathogenesis of other diseases," the editorialists concluded.