Thrombocytosis is a common finding with a wide range of primary and secondary causes. Most cases are reactive and a bone marrow (BM), if performed, shows megakaryocytic hyperplasia with normal differentiation. Primary thrombocytosis may be due to a myeloproliferative or myelodysplastic/myeloproliferative neoplasm or myelodysplastic syndrome. In essential thrombocythaemia there is isolated megakaryocytic hyperplasia with large or giant forms with hyperlobated nuclei. Erythropoiesis, granulopoiesis and reticulin are normal. Polycthaemia vera may present with a marked thrombocytosis. The BM shows a pan-myelosis (normal erythroid and granulocytic differentiation). Megakaryocytes are commonly clustered, pleomorphic and have a high nuclear:cytoplasmic ratio. Primary myelofibrosis can present with a marked thrombocytosis and the BM has variable cellularity and spatial distribution. Megakaryocytes are markedly abnormal: paratrabecular, intrasinusoidal, clusters, atypical morphology (large, distorted, high nuclear:cytoplasmic ratio, poorly lobated nuclei). Reticulin is increased and may be accompanied by collagen and/or new bone formation. In refractory anaemia with ringed sideroblasts associated with marked thrombocytosis the BM shows increased megakaryocytic hyperplasia with morphological features similar to ET or PMF plus ring sideroblasts. Myelodysplastic syndrome with isolated del(5q) is commonly associated with thrombocytosis. The BM is normocellular or mildly hypercellular with dyserythropoiesis. Megakaryocytes are small with eccentrically placed hypolobated nuclei.