Our objective was to evaluate left colonic motility patterns recorded under physiological conditions during 24 hr in fully ambulant nonconstipated IBS patients compared to healthy controls. A 42-hr manometry of the left colon was performed in 11 nonconstipated IBS patients and 10 age- and sex-matched healthy volunteers. On day 1, a 6-channel, 10-cm interval, solid-state catheter was positioned. Frequency, amplitude, and motility index (MI) of segmenting pressure waves in the descending and sigmoid colon were calculated during the 24-hr study period on day 2. High-amplitude propagated contractions (HAPCs) were identified visually and their characteristics were calculated. In IBS patients a higher frequency of segmenting pressure waves was observed in the sigmoid colon compared to the descending colon (P = 0.006). In contrast, no regional differences were observed in controls. Awakening (P = 0.048) as well as having a meal (P = 0.024) was associated with a smaller increase of contraction frequency in the descending colon of IBS patients compared to controls. HAPCs occurred more frequently in IBS patients than in controls (P = 0.035). HAPCs in IBS patients reached a more distal colonic level and occurred more frequently in clusters. Defecation in IBS patients, but not in controls was always preceded by a cluster of HAPCs. In conclusion, left colonic segmenting pressure waves and HAPC characteristics are altered in nonconstipated IBS patients.