The present invention relates to the use of the Chemokine (C—C motif) ligand 20 (CCL20) as a biomarker to stratify or identify populations of patients suffering from interleukin-23 (IL23)-mediated diseases (e.g., Crohn's disease) responsive to treatment with an, anti-IL23 antagonist (including, e.g., anti-IL23 antibodies). Levels of CCL20 above or below a predetermined threshold can be used, for example, (i) to determine whether a patient with an IL23-mediated disease or disorder such a Crohn's disease is eligible or non-eligible for treatment with a therapeutic agent, (ii) to determine whether treatment with a certain agent should be commenced, suspended, or modified, (iii) to diagnose whether the IL23-mediated disease is treatable or not treatable with a specific therapeutic agent, or (iv) to predict the outcome of treating the IL23-mediated disease with a specific therapeutic agent. CCL20 can be used in combination with other IL23 pathway biomarkers such as IL22 and/or lipocalin-2 (LCN2).