Idiopathic myelofibrosis is a chronic myeloproliferative disorder in which the characteristic fibroblast proliferation is thought to be a secondary phenomenon resulting from the inappropriate release of megakaryocyte- and/or monocyte-derived growth factors, including PDGF, TGF-β, bFGF and calmodulin. In contrast, the haematopoietic cells are clonal, although the underlying pathogenetic mechanisms remain essentially unknown. Cytogenetic studies have highlighted that 13q–, 20q–, +8 and abnormalities of chromosomes 1, 7 and 9 constitute more than 80% of the chromosomal changes. A third of idiopathic myelofibrosis cases have abnormal karyotypes at diagnosis, a figure that increases if follow-up analyses are performed. Evolution to more complex karyotypes may accompany clinical progression, with abnormalities increasing to around 90% following acute leukaemic transformation. Cytogenetic abnormalities have been associated with prognosis and to a lack of treatment response to androgens. Oncogene mutations are rare and include point mutations in N-RAS, c-KIT and TP53.