Background: Acupuncture is used by patients as a treatment for irritable bowel syndrome (IBS) but the evidence on effectiveness is limited. The purpose of the study was to evaluate the effectiveness of acupuncture for irritable bowel syndrome in primary care when provided as an adjunct to usual care. Methods: Design: A two-arm pragmatic randomised controlled trial.Setting: Primary care in the United Kingdom.Patients: 233 patients had irritable bowel syndrome with average duration of 13 years and score of at least 100 on the IBS Symptom Severity Score (SSS).Interventions: 116 patients were offered 10 weekly individualised acupuncture sessions plus usual care, 117 patients continued with usual care alone.Measurements: Primary outcome was the IBS SSS at three months, with outcome data collected every three months to 12 months.Results: There was a statistically significant difference between groups at three months favouring acupuncture with a reduction in IBS Symptom Severity Score of -27.43 (95% CI: -48.66 to -6.21, p = 0.012). The number needed to treat for successful treatment (≥50 point reduction in the IBS SSS) was six (95% CI: 3 to 17), based on 49% success in the acupuncture group vs. 31% in the control group, a difference between groups of 18% (95% CI: 6% to 31%). This benefit largely persisted at 6, 9 and 12 months. Conclusions: Acupuncture for irritable bowel syndrome provided an additional benefit over usual care alone. The magnitude of the effect was sustained over the longer term. Acupuncture should be considered as a treatment option to be offered in primary care alongside other evidenced based treatments.Trial Registration: Current Controlled Trials ISRCTN08827905. © 2012 MacPherson et al.; licensee BioMed Central Ltd.