While cognitive-behavioral therapy for IBS is quite effective, the limited availability of competent therapists and lack of access to treatment remain problematic. This paper reports on a small, randomized, controlled trial of a five week internet based cognitive-behavioral intervention for IBS with limited therapist feedback via e-mail. Fifty-four IBS patients were recruited via the internet and randomly assigned to either immediate treatment or a wait-list control group. Thirty-one subjects completed the post-treatment assessment. 77% of treatment completers also completed a 3-month follow-up assessment. Treatment completers experienced statistically and clinically significant declines in IBS symptoms and improvements in quality of life. Those gains were substantially maintained at follow-up. Treatment efficacy was partially mediated by reductions in the tendency to catastrophize the social and occupational implications of symptoms, suggesting that catastrophizing may be an important target for treatment.