ALTHOUGH phantom-limb pain is a frequent consequence of the amputation of an extremity, little is known about its origin^1-4. On the basis of the demonstration of substantial plasticity of the somatosensory cortex after amputation⁵ or somatosensory deafferentation in adult monkeys⁶, it has been suggested that cortical reorganization could account for some non-painful phantom-limb phenomena in amputees and that cortical reorganization has an adaptive (that is, pain-preventing) function^2,5,7,8. Theoretical and empirical work on chronic back pain^9,10 has revealed a positive relationship between the amount of cortical alteration and the magnitude of pain, so we predicted that cortical reorganization and phantom-limb pain should be positively related. Using non-invasive neuromagnetic imaging techniques to determine cortical reorganization in humans^11-13, we report a very strong direct relationship (r = 0.93) between the amount of cortical reorganization and the magnitude of phantom limb pain (but not non-painful phantom phenomena) experienced after arm amputation. These data indicate that phantom-limb pain is related to, and may be a consequence of, plastic changes in primary somatosensory cortex.