OBJECTIVES:Our research group has developed an internet-delivered cognitive behavioral treatment (ICBT) for irritable bowel syndrome (IBS). We compared ICBT with internet-delivered stress management (ISM) for IBS to assess whether the effects of ICBT are specific. METHODS: This was a randomized controlled trial, including 195 self-referred participants diagnosed with IBS. The treatment interventions lasted for 10 weeks and included an online therapist contact. The ICBT emphasized acceptance of symptoms through exposure to IBS symptoms and related negative feelings. The ICBT also included mindfulness training. The ISM emphasized symptom control through relaxation techniques, dietary adjustments, and problem-solving skills. Severity of IBS symptoms was measured with the gastrointestinal symptom rating scale-IBS version (GSRS-IBS). Credibility of the treatments and expectancy of improvement were assessed with the treatment credibility scale. The participants' perceived therapeutic alliance with their online therapist was measured with the working alliance inventory. RESULTS: At post-treatment and 6-month follow-up, 192 (99%) and 169 (87%) participants returned data, respectively. At post-treatment and 6-month follow-up, we found significant differences on the GSRS-IBS, favoring ICBT. The difference on GSRS-IBS scores was 4.8 (95% confidence interval (CI): 1.2-8.4) at post-treatment and 5.9 (95% CI: 1.9-9.9) at 6-month follow-up. There were no significant differences on the treatment credibility scale or the working alliance inventory between the groups. CONCLUSIONS: Internet-delivered CBT has specific effects that cannot be attributed only to treatment credibility, expectancy of improvement, therapeutic alliance, or attention. Furthermore, a treatment based on exposure exercises specifically tailored for IBS may be a better treatment option than general stress and symptom management for IBS patients. ICBT is a promising treatment modality for IBS as it can be offered to IBS patients in much larger scale than conventional psychological treatments.