Whether the pathogenesis of cluster headache (CH) is peripheral or central is still matter of debate. An involvement of central structures related to pain perception and modulation, which also causes an alteration of the physiological pattern of pain perception in CH, has been hypothesized. We investigated the pattern of brain response to pain in normal subjects and CH patients by evaluating the cerebral blood flow (CBF) changes using an experimental model of tonic aching pain stimulation, the cold water pressor test (CWPT). CBF was assessed quantitatively by the Xe-133 inhalation method and single photon emission tomography (SPET), at rest and during CWPT, as previously described (Di Piero et al., 1994). CWPT was performed in 12 volunteers and in seven patients with CH. All the CH patients had a left-sided headache and were studied in a headache-free phase out of the cluster period. During CWPT, volunteers showed a significant CBF increase in the contralateral primary sensorimotor (P < 0.001), frontal (P < 0.01) and temporal (P < 0.002) regions and thalamus (P < 0.01) and in the ipsilateral temporal (P < 0.005) and anterior cingulate (P < 0.01) regions. During left-hand stimulation (ipsilateral to the headache side) by CWPT in CH patients, CBF changes were significantly lower than those observed in volunteers in the contralateral primary sensorimotor region (P < 0.0005) and thalamus region (P < 0.01). There were no significant differences in the brain response observed during the stimulation of the hand contralateral to the headache side. In conclusion, in a headache-free phase out of the cluster period, the pattern of cerebral activation during tonic pain stimulation of the hand ipsilateral to the headache side is critically modified in CH patients in areas which are probably involved in the detection of the stimulus intensity. This modification may reflect a marker of a biological modification of the pain conveyance system. The fact that it is also present out of the active period of the disease, suggests a possible involvement of central tonic pain mechanisms in the pathogenesis of CH.