In our report, myelofibrosis in children is discussed and two cases of acutely developing myelofibrosis in association with acute megakaryoblastic leukaemia (M7) are presented. In the first case (girl, 34 months), it was acute myelofibrosis of hypocellular marrow. Diagnosis of M7 was confirmed by positive reaction of blasts from peripheral blood with CD42 and CD61 monoclonal antibodies. In the other child (girl, 5 years old), Ph1(+) chronic myeloid leukaemia diagnosed 22 months earlier transformed to M7. Similar to the first case, no marrow aspirate could be obtained and the diagnosis of M7 was made by the bone marrow histology that showed the presence of grossly fibrosed, hypercellular marrow with a large number of dysplastic, maturing megakaryocytes. Neither of the children had Down's syndrome. Although according to FAB classification both cases represent the same haematological entity, their clinical and histopathological presentations are very different.