The UK Biobank is an ongoing, large-scale, population-based study designed to examine determinants of health in middle and old age . Besides extensive collection of health questionnaire data, biological samples and physical measurements, it has incorporated cardiovascular magnetic resonance (CMR) imaging–the gold standard for analysis of cardiac structure and function–to provide detailed imaging phenotypes . At present, there is a paucity of data on the effects of MHT on left ventricular (LV) and left atrial (LA) volumes and function, alterations in which are markers of subclinical cardiovascular disease and have prognostic implications.