The study will recruit 80 out-patients diagnosed with Irritable Bowel Syndrome from a gastroenterological clinic. The recruitment will be consecutive with all patients receiving the diagnosis being offered to participate. After recruitment and 3 week baseline measure of IBS symptoms they will be randomised to either 10 weeks of internet delivered CBT or waiting list. After treatment IBS-symptoms are measured again to assess treatment effectiveness. All patients on waiting list are offered the same treatment as patients randomised to treatment.

All patients are assessed 12 months after completion of treatment.

Patients in treatment condition are hypothesized to experience significant reduction IBS symptoms and quality of life compared to patients on waiting list.

Inclusion Criteria:

• Clinical diagnosis of Irritable Bowel Syndrome according to Rome III criteria

Exclusion Criteria:

• History of inflammatory bowel disease (IBD)
• Symptom debut after age of 50
• Ongoing severe mental illness (psychosis, severe depression, suicidal ideation, bipolar disease)
• Inability to participate in internet delivered treatment