Irritable Bowel Syndrome (IBS) is a chronic, functional gastrointestinal (GI) disorder. Cognitive behavioral therapy (CBT) has been shown to be efficacious for IBS; however, access to qualified CBT therapists is limited. This study was a randomized, controlled trial of a CBT self-help workbook for IBS. Sixty patients with IBS were randomly assigned to active treatment or waitlist control. Participants were evaluated 6 weeks post-randomization at which point waitlist controls crossed over to active treatment. Treatment completers (17 in the immediate treatment group vs 19 in the waitlist control) experienced statistically and clinically significant improvement in quality of life, GI symptom severity, visceral sensitivity and catastrophic cognitions immediately post-treatment. Treatment efficacy was partially mediated by reductions in catastrophic thinking and visceral sensitivity. Limitations of the study included a high attrition rate, as is common in both internet-based and self-help intervention studies. Multiple imputation of missing data for post-treatment was carried out to correct for attrition, and subsequent analyses replicated prior statistically significant improvement on all major outcome measures. This study represents a move towards wider dissemination of and access to empirically-supported treatments for IBS, as well as an effort to test a self-help book in a randomized, controlled trial prior to publication.