Since its first description 30 years ago, dedifferentiated chondrosarcoma has been the prototype of all dedifferentiated sarcomas. The presence of two tumour portions of different mesenchymal differentiation lineages in these neoplasms gives rise to three key questions, which are on the way to being resolved. Does it split up? And if so, how does it split up and when does it split up? Accumulating data provide evidence for a common monoclonal origin of both tumour portions and suggest that dedifferentiated chondrosarcoma is a paradigmatic neoplasm of mesenchymal transdifferentiaton in vivo. Two categories emerge of dedifferentiated chondrosarcomas with different cell biology: the classical one, with a low-grade chondroid component splitting up late, and a second type, with a high-grade chondroid component splitting up early in tumour development. Copyright © 1999 John Wiley & Sons, Ltd.