The thermal grill illusion of pain (TGIP) is a paradoxical burning pain sensation elicited by the simultaneous application of innocuous cutaneous warm and cold stimuli with a thermode ("thermal grill") consisting of interlaced heated and cooled bars. Its neurophysiological mechanisms are unclear, but TGIP may have some mechanisms in common with pathological pain, including central sensitization in particular, through the involvement of N-methyl- d -aspartate receptors. However, few studies have investigated TGIP in patients with chronic pain and its clinical relevance is uncertain. In this presentation we will summarized recent data regarding the TGIP in patients with in patients with fibromyalgia or irritable bowel syndrome, which are regarded as typical "nociplastic" primary pain syndromes related to changes in central pain processing. Our results showed that TGIP was increased in fibromyalgia and to a lesser extent in IBS patients and was correlated with clinical pain suggesting that TGIP might reflect pathophysiological mechanisms such as central sensitization.