Inter- and intraindividual variation in pressure-volume relations of the rectum in normal subjects and patients with the irritable bowel syndrome. The relation between intrarectal volume and pressure during increasing rectal distension by a latex balloon were studied on repeated occasions in 10 healthy adult volunteers to define variations within and between individuals. A wide intersubject variation in the maximum tolerable volume (58-908 ml) and pressure (12.2-108.8 cm H2O) at this end point was seen, and these two values were correlated (r = 0.78). Intrasubject variation in maximum tolerable volume also occurred which was related to study order and progressively reduced with repeated study. In 26 unselected patients with pain predominant irritable bowel syndrome similar intersubject variation was noted and virtually all patients data fell within the calculated 95% confidence limits of the normal individuals. Differentiation between patients and normal subjects was not possible from knowledge of rectal responses. These noticeable inter- and intrasubject variations in rectal responses to distension need to be considered whenever similar techniques are proposed for use in the study of rectal disease or of rectal response to treatment. 