Hypnosis improves irritable bowel syndrome (IBS), but the mechanism is unknown. Possible physiological and psychological mechanisms were investigated in two studies. Patients with severe irritable bowel syndrome received seven biweekly hypnosis sessions and used hypnosis audiotapes at home. Rectal pain thresholds and smooth muscle tone were measured with a barostat before and after treatment in 18 patients (study I), and treatment changes in heart rate, blood pressure, skin conductance, finger temperature, and forehead electromyographic activity were assessed in 24 patients (study II). Somatization, anxiety, and depression were also measured. All central IBS symptoms improved substantially from treatment in both studies. Rectal pain thresholds, rectal smooth muscle tone, and autonomic functioning (except sweat gland reactivity) were unaffected by hypnosis treatment. However, somatization and psychological distress showed large decreases. In conclusion, hypnosis improves IBS symptoms through reductions in psychological distress and somatization. Improvements were unrelated to changes in the physiological parameters measured.