The authors present a case of giant pulmonary metastasis from cystosarcoma phyllodes of the breast, and correlate the CT and MR findings with the pathological findings. Although CT showed homogenous low density in most parts of the tumor, T 2-weighted MR images presented a more variegated appearance, namely, peculiar slit-like areas of low intensity between areas of high intensity. The MR findings reflected the characteristic gross pathology of cystosarcoma phyllodes, showing narrow cavities filled with bloody fluid surrounding intracystic protruding tumors with abundant necrotic change.