Patients with irritable bowel syndrome (IBS) report an increased frequency of sensations during rectal distension in comparison with healthy subjects. This alteration might be due to a psychological response bias leading patients to over report their sensations. The aim of this study was to measure perceptual sensitivity and response bias during rectal distension in healthy subjects and IBS patients using the sensory decision theory (SDT). Thirteen healthy subjects and 22 IBS patients underwent five rectal distensions up to 100 mL, five up to 200 mL and five sham distensions. They were asked to identify the distension by means of an electronic marker. Perceptual sensitivity and response bias were calculated according to the SDT. The patients identified a more 100 mL distensions than the healthy subjects (P = 0.02), whereas there was no difference in the number of identified 200 mL and sham distensions between the two groups. The perceptual sensitivity of IBS patients was significantly greater during 100 mL (P = 0.01), but not during 200 mL distensions. The response bias was not significantly different between the two groups. These data suggest that the increased frequency of sensations reported by IBS patients is not due to a psychological response bias.