Every year, millions of people suffer from acute back pain. For some people, the pain persists for weeks (subacute pain) and then resolves, while others go on to develop long-term, debilitating chronic pain. Who will make that transition and how it happens remain mysterious. Now, using brain imaging, A. Vania Apkarian and his colleagues at Northwestern University, Chicago, US, have identified structural differences in the brain's white matter—the bundles of axons that carry communications between neurons throughout the brain—in people whose pain persisted compared to those who recovered. If replicated in larger studies, the structural discrepancy may provide a biomarker of vulnerability to chronic pain. The group presented their findings in October at the 2012 annual meeting of the Society for Neuroscience in New Orleans, US.