Upper small bowel motility was recorded for more than 30 h in each of 22 patients with the irritable bowel syndrome (IBS) and in two control groups, one consisting of 10 healthy volunteers and the other of 5 patients with inflammatory bowel disease. When subjects underwent a long period of intermittent mental stress, one or more of three motor abnormalities occurred in 19 patients; and only one abnormality was seen in 1 control (p less than 0.0001). These abnormalities were the total abolition of migrating motor complexes under stress, and abnormal irregular contractile activity, which was either spontaneous or evoked by stress. Sometimes the irregular contractile activity coincided with the occurrence of typical IBS symptoms. Males predominated (p = 0.05) among those with spontaneous irregular motor activity, who tended to have more symptoms (p = 0.014) than did those affected only by stress. The data suggest that irritable bowel syndrome is a paroxysmal motor disorder which may be detected in the small bowel and which may be either spontaneous or evoked.