The present invention provides a diagnostic method which may be used to determine the likelihood that a subject with Irritable Bowel Syndrome (IBS) will respond to treatment with an IBS intervention diet or faecal microbiota transplant (FMT). In particular, the method may be used to predict, or determine the likelihood of, a positive response of the subject with IBS to treatment with an IBS intervention diet or FMT, especially to determine the likelihood that the dietary intervention or FMT may have a positive (i.e. beneficial) effect on the subject's GI tract, specifically the GI tract microbiota, or other symptoms or complications of IBS (e.g. reducing severity thereof). The method of the present invention is based on analysing the abundance of certain bacteria in GI tract samples, e.g. by nucleic acid analysis.