A randomized, double-blind trial of a psyllium preparation was initiated in 77 patients with painful irritable bowel syndrome. Sixty-patients finished and submitted symptom data for 8 weeks while taking placebo (n = 34) or psyllium (n = 26). Increase in normal stools and decrease in pain severity (p less than 0.05) occurred equally in both groups. Subjective improvement was reported by 24 of 34 patients on placebo and 20 or 26 on psyllium (p greater than 0.05). Five symptom variables were significantly correlated (p less than 0.05) with patient's subjective global assessment (R = 0.64). Discriminant analysis of Minnesota Multiphasic Personality Inventory variables yielded overall rates of correct prediction of 66.1% for whether patients got "much better" and 77.9% for whether they voluntarily dropped from the study. A major placebo effect occurs in patients with painful irritable bowel syndrome and is probably responsible for the efficacy of psyllium. Personality factors influence the magnitude of therapeutic response and whether patients discontinue treatment within 8 weeks.