Studies of rectal sensory thresholds and compliance in patients with the irritable bowel syndrome have produced conflicting results though there is persistent evidence of rectal hypersensitivity particularly in those with diarrhoea-predominant symptoms. This study examined rectal sensation and compliance in 31 patients with constipation-predominant irritable bowel syndrome (mean age 41 years, 27 female) and 17 healthy volunteers (mean age 45 years, 17 female). A rectal balloon was inflated with fluid at a constant rate and the volume and intrarectal pressure at sensory threshold was recorded. The volumes at first (129 +/- 8 vs 229 +/- 24 ml, P < 0.001 Mann-Whitney-U test), constant (159 +/- 12 vs 286 +/- 21, P < 0.001) and maximum tolerated sensation (290 +/- 13 vs 509 +/- 19, P < 0.001) were all significantly less in the irritable bowel group. There was no significant difference in intrarectal pressures at any of these volumes (29.0 +/- 2.2 cmH2O vs 29.0 +/- 2.5, 35.0 +/- 2.5 vs 34.0 +/- 2.8, 71 +/- 2.5 vs 65.0 +/- 3.0 respectively). Rectal compliance was significantly less in the irritable bowel group (3.6 +/- 0.2 ml/cmH2O vs 8.7 +/- 0.4, P < 0.001). Twenty two patients complained of abdominal pain on balloon inflation, mimicking that experienced as part of their symptoms. Patients with constipation-predominant irritable bowel syndrome have rectal hypersensitivity and reduced compliance.