Electron spin resonance (ESR) was used to examine human keloid, normal connective tissue and skin. The results showed that more oxygen free radicals were identified in keloid, less in normal connective tissue while no free radicals were found in skin. The author discussed the significance of the occurrence of oxygen free radicals and its relation with keloid and suggested that reduction or elimination of production of oxygen free radicals by its scavenger may be of help in preventing the excessive growth of scar tissue.