BACKGROUND: Psychotherapy is effective in treating irritable bowel syndrome, but the effect of relaxation training, a brief psychological group intervention, is not known. AIM: To determine the efficacy of relaxation training in a large cohort of irritable bowel syndrome patients. METHODS: Ninety-eight irritable bowel syndrome patients were included in this randomized controlled trial. Forty-six patients received standard medical care (CON) and 52 received four 90-min sessions of relaxation training in small groups in addition to standard medical care. Irritable bowel syndrome symptom severity, medical consumption and quality of life were assessed at baseline in patients and in 38 healthy controls and evaluated in patients at 3, 6 and 12 months after intervention. RESULTS: Irritable bowel syndrome symptom severity was significantly reduced in the relaxation training group compared to CON at 3, 6 and 12 months after treatment (time-by-treatment interaction, P = 0.002). The number needed to treat for long-term improvement was 5. Quality of life had improved (general health, P = 0.017; health change, P = 0.05). Frequency of doctor visits was reduced (P = 0.039). CONCLUSIONS: Relaxation training is a brief group intervention that significantly improves symptom severity, general health perception and medical consumption in irritable bowel syndrome patients immediately after, as well as 6 and 12 months after intervention.