BACKGROUND/AIMS: The differential diagnosis of constipation-predominant irritable bowel syndrome (IBS-C) and functional constipation (FC) is difficult in many cases, even though the suggested pathophysiologies are quite different. The presence of visceral hypersensitivity, which is frequent in diarrhea-predominant irritable bowel syndrome, is still controversial in IBS-C. The aim of this study was to evaluate the differences in rectal visceral sensitivity to rectal distension in patients with IBS-C and FC. METHODS: In thirty eight patients with FC and 31 patients with IBS-C, as accessed by the Rome II criteria, rectal visceral sensitivity and compliance were measured by a barostat examination. Pressure thresholds of the first sense, urgency, maximum tolerable volume and the sensitivity index percentage were compared in both group according to the gender. RESULTS: for the female patients, the sensitivity index percentage of the IBS-C was significantly lower than that of FC (p<0.05). for the male patients, the first sense threshold pressure was significantly lower in the IBS-C patients than in the FC patients, (p<0.05). CONCLUSIONS: The patients with IBS-C showed more rectal visceral hypersensitivity compared with the FC patients irrespective of the gender. Male patients with IBS-C showed visceral hypersensitivity to low-grade stimuli; in contrast, the female patients with IBS-C showed hypersensitivity to high-grade stimuli, suggesting a differential pathophysiologic mechanism of visceral hypersensitivity in the different gender patients with IBS-C.