Evidence that CFS truly does deserve all three elements of its name has accumulated over the years but a definitive diagnostic test has remained elusive. Until, perhaps, now. For in this week’s Proceedings of the National Academy of Sciences Robert Naviaux of the University of California, San Diego, and his colleagues published evidence that the metabolisms of those diagnosed with CFS are all changing in the same way. Their data suggest it is this cellular response to CFS-triggering traumas, and not the way the response is set in motion, which should define the illness. They also show that this response produces a chemical signal that might be used for diagnosis.