OBJECTIVES: To investigate the variability in cerebral activation according to pain intensity and the association between variability in cerebral activation and clinical features in patients with fibromyalgia syndrome (FMS) using functional magnetic resonance imaging (fMRI). METHODS: Nineteen FMS female patients and 22 age-matched healthy female controls were enrolled in this study. Changes in cerebral activation area were measured using blood oxygenation level-dependent (BOLD) contrast fMRI after application of both medium and high pressure stimuli to the left thumbnail bed. RESULTS: We identified the insular cortex (IC) and superior temporal gyrus (STG) as regions of interest (ROIs) in this analysis. Cerebral activation at the bilateral IC in response to high pressure stimuli was significantly greater in FMS patients than it was in the controls, whereas there were no differences in BOLD signal changes in the STG regions between FMS patients and controls, irrespective of pain level. Prominent signal changes at both ROIs in FMS patients were noted between high and medium pressure (p<0.001 contralateral IC, p=0.001 for ipsilateral IC, p=0.008 for contralateral STG, and p=0.049 for ipsilateral STG). BOLD signal changes on the contralateral STG after medium stimuli were correlated with tender point count (r=0.586, p=0.013). CONCLUSIONS: This study revealed more distinct signal variability in the ICs in FMS patients than in those of controls in response to high pressure stimuli. The IC can therefore be considered to be a region susceptible to pain perception in FMS patients.