Bone marrow-derived cells include haematopoietic cell lineages and the recently described endothelial progenitor cells (EPCs). It has been recently emphasised that these marrow-derived cells contribute to tumour angiogenesis, and different mechanisms have been proposed that account for this activity. Whereas haematopoietic cells may promote tumour angiogenesis through the release of proangiogenic factors or by creating permissive conditions in the tumour microenvironment that favour the growth of locally derived blood vessels ("paracrine" role), endothelial progenitors are thought to directly incorporate into nascent blood vessels as bona fide endothelial cells ("building block" role). The relative contribution of these distinct pathways to tumour angiogenesis is the subject of intense investigation and debate.