Empirical studies indicate that alexithymia exacerbates physical illness. However, direct evidence to explain the mechanism of this exacerbation has not been provided. One hypothesis is that alexithymics amplify unpleasant internal signals. In the present study, we investigated how alexithymia influences sensitivity to visceral stimulation in human. In 45 non-clinical healthy subjects (34 males and 11 females), brain processing of visceral sensation induced by colonic distension was examined using H(2)(15)O positron emission tomography (PET). Subjective feeling evaluated on an ordinate scale and neuroendocrine response to stimuli were also measured. The degree of alexithymia was determined using the 20-item of Toronto alexithymia scale (TAS-20), and the correlation between reaction to stimuli and the scores of TAS-20 and its three subscales [difficulty to identify feelings (DIF), difficulty to describe feelings (DDF) and external oriented thinking (EOT)] was evaluated. Greater activation was observed during colonic distension in the pregenual anterior cingulate cortex, right insula and midbrain in the 10 (out of 45) subjects that were identified as alexithymic by TAS-20 scores larger than 61. TAS-20 scores positively correlated with both activity in the right insula and orbital gyrus and adrenaline levels in the blood in response to stimulation. Subjects with high scores of DIF perceived strong pain, urgency for defecation, stress, anxiety, and slight sleepiness. The present study demonstrates that alexithymia is associated with hypersensitivity to visceral stimulation. This finding supports the somatosensory amplification hypothesized in alexithymics and is important to elucidate the influence of alexithymia on brain-gut function, particularly to understand the pathophysiology of FGIDs (functional gastrointestinal disorders).