This is the first time a report is being published in medical literature on the MR imaging of a cystosarcoma phylloides tumour after intravenous contrast agent administration. MR tomography enabled complete visualisation of the tumour even in the region close to the thoracic wall, as well as clear delineation against the non-tumorous glandular tissue. This means that the limits along which complete extirpation of the tumour could be performed, were determined accurately before the operation, such complete removal being imperative to avoid tumour recurrence.