A retrospective study of 21 patients with acute transformation of chronic granulocytic leukaemia (CGL) and myelofibrosis was undertaken. The patients were divided into 3 groups according to whether transformation had occurred in pre-existing CGL (11 patients), myelofibrosis (6 patients) or in patients who presented with acute leukaemia and significant marrow fibrosis--acute myelofibrosis (4 patients). In an attempt to investigate some recent reports that during the acute blastic crisis there is conversion to the haemopoietic stem cell, the morphology of the blast cell and the response to therapy were assessed. The blast cell morphology was reviewed in 17 of the 21 patients studied. Three patients had blast cells which morphologically resembled lymphoblasts. The mortality in these patients was similar to that in patients in whom the blast cell morphology was myeloblastic. The response to treatment in the acute phase was poor. However, some patients who received therapy with thioguanine, daunorubicin, cytosine arabinoside, methotrexate, prednisone, cyclophosphamide and vincristine (TRAMPCO) appeared to do better. in most patients, splenectomy was performed for advanced diseases. Acute blastic transformation supervened within 4 months in 3 of 4 patients with CGL who underwent splenectomy. In 3 of 5 patients with myelofibrosis, splenectomy was followed by an increase of primitive cells in the peripheral blood and rapid liver enlargement. Although they had improved symptomatically the patients died within 10 months of the splenectomy. The indications for and timing of splenectomy remain controversial.