We herein report a case of huge lymphatic and venous malformations of the mediastinum (LVM). A 19-year-old man presented for evaluation of a mediastinal mass. On chest computed tomography, the mass demonstrated contrast enhancement and showed dilated veins draining into the superior and inferior vena cava, with multiple scattered calcifications. The lesion was enhanced heterogeneously on T1-weighted magnetic resonance imaging (MRI) and hyperintense on T2-weighted MRI. Contrast-enhanced MRI revealed that the mass was enhanced, with the multilocular part marginally enhanced. From these images, we diagnosed him with LVM. Given that an operation presented a high risk, we decided to follow him up without any treatment.