Irritable bowel syndrome (IBS) patients in Western countries usually manifest autonomic nerve dysfunctions and abnormal psychological behaviors. The purpose of this study was to assess whether Oriental IBS patients with predominant bowel symptoms also exhibited similar abnormalities. We enrolled 40 IBS patients from the outpatient clinic and 20 controls with normal daily bowel habit for study. The IBS patients were further divided according to their predominant bowel habit: 20 were constipation-predominant and 20 were diarrhea-predominant. Sympathetic function was evaluated by sympathetic skin response (SSR) while vagal cholinergic function was determined by measuring R-R interval variation (RRIV) in electrocardiography during rest and deep breathing. Psychological parameters were assessed by scales of the Minnesota Multiphasic Personality Inventory (MMPI) and the Hopkins Symptom Checklist (HSCL-90). IBS patients, despite their bowel habit, showed normal SSR response. RRIV during deep breathing was significantly lower in constipation-predominant IBS patients than in controls or diarrhea-predominant IBS patients (16.5+/-3.1% vs 20.5+/-4.8% and 21.5+/-4.6%, P < 0.001). IBS patients also exhibited abnormal MMPI measuring scores on depression, hysteria, paranoia, and masculinity/femininity scales. In addition, they also had more severe psychological distress in the items of HSCL-90 measurement. In conclusion, vagal dysfunction characterizes Oriental constipation-predominant IBS patients seeking medical help. Abnormal psychoneurotic profiles also exist in these IBS patients, irrespective of their bowel habits.