This data dictionary describes the CLUSTER MTX clinical dataset. It contains key variables of interest for measuring treatment response to methotrexate (MTX) in juvenile idiopathic arthritis (JIA) patients at baseline/timepoint 1 (start of MTX treatment or up to 3 months before) and timepoint 2 (6 months after baseline, within a 3-12 month window). If a patient had multiple visits within a timepoint window, the one closest to baseline/6 months was selected. There are 2899 patients in this dataset. CLUSTER has brought together data from 4 UK observational JIA research cohort studies: the UK JIA Biologics Register, the Childhood Arthritis Prospective Study (CAPS) and the Childhood Arthritis Response to Medication Study (CHARMS). Clinical data from these studies was pooled to create the CLUSTER MTX clinical dataset. More details of CLUSTER's clinical data harmonisation process are available here: <link> We are open to collaborating and sharing data with researchers who have research questions that may be answered by our data. Access to CLUSTER data is regulated according to the conditions of patient consent, study ethics and the CLUSTER Consortium’s policies. Access is available to researchers via an application to the CLUSTER data access committee - more details are available on CLUSTER's website.