The chronic myeloproliferative disorders (MPD) are well characterized clinical entities comprising polycythaemia vera (PV), idiopathic thrombocythaemia (IT) (also called megakaryocytic myelosis, mature type; MegM), chronic myeloid leukaemia (CML), and myelofibrosis/osteomyelosclerosis (MF/OMS). Three thousand one hundred and eight bone biopsies of 2629 patients with established MPD were examined to investigate the histologic features of MPD in a large material in order to identify criteria for the histologic classification and differential diagnosis of these disorders. Detailed histologic characteristics were defined for each of the disorders and the results showed that in the majority of cases MPD may be recognized and classified by the initial bone marrow histology. Utilizing the predominant proliferative cell(s) in the bone marrow, PV was categorized into 4 types: 1. the classic, tri-linear type; 2. a bi-linear type with hyperplasia of the erythroid and granulocytic lines; 3. a bi-linear type with hyperplasia of the erythroid and megakaryocytic cell lines and 4. a uni-linear type with isolated increased erythrocytic proliferation. CML showed 2 sub-divisions: 1. the granulocytic, uni-linear type and 2. the bi-linear type with proliferation of myeloid and megakaryocytic lines. The former had a tendency to evolve into blastic crisis, while the latter was prone to develop into MF/OMS. It was primarily uni-linear exhibiting increased megakaryocytes. In most cases, MF/OMS was shown (by means of follow-up biopsies) to arise out of those entities of MPD which had included megakaryocytic hyperplasia and to which the proliferation of fibroblasts was secondary. The conclusion is drawn that an initial bone marrow biopsy provides additional diagnostic and prognostic data in this group of haematologic malignancies.