Liposarcoma rarely occurs in the mediastinum, and most reports predate the current genetically based classification system. We report the clinicopathologic and molecular genetic features of a series of thoracic liposarcomas identified over a 60-year period. Twenty-four confirmed cases were reclassified using the most recent World Health Organization classification. Fluorescent in situ hybridization for CPM amplification and/or DDIT3 rearrangement was performed on selected cases. The 24 cases occurred in 13 men and 11 women (mean age, 53 y; range, 15 to 73 y) and arose in all mediastinal compartments. All subtypes were encountered with 8 well-differentiated liposarcomas, 6 dedifferentiated liposarcomas (3 of 6 confirmed CPM+), 7 pleomorphic liposarcomas (2 of 7 confirmed CPM-, 1 of 7 confirmed DDIT3-), 2 myxoid liposarcomas, and 1 unclassifiable liposarcoma (CPM- and DDIT3-). Unusual histologic features included myxoid well-differentiated liposarcoma mimicking myxoid liposarcoma (2 cases), lipoleiomyosarcoma (1 case), dedifferentiated liposarcoma with "meningothelial"-like dedifferentiation, differentiated myxoid liposarcoma mimicking well-differentiated liposarcoma (CPM-), and pleomorphic liposarcoma with epithelioid and myxoid change. Follow-up information was available for 19 patients (mean, 55 mo; range, 8 to 252 mo). Outcome was strongly associated with histologic subtype, with death from disease occurring in 1 of 6 well-differentiated, 1 of 4 dedifferentiated, 5 of 7 pleomorphic, and 2 of 2 myxoid liposarcomas. The mediastinum shows a preponderance of uncommon subtypes and unusual morphologic variants. Correct classification has important implications, with most patients with well-differentiated/dedifferentiated liposarcoma having a protracted clinical course, in contrast to the more rapid disease progression seen in patients with myxoid and pleomorphic liposarcoma.